Three orders, one box: the setup
Three merges from our own orders between 2026-05-14 and 2026-07-30 produced £4.80 saved, £7.40 lost, and £6.10 saved on freight that later turned into a £34.20 write-off. Same warehouse, same pricing ladder, same person making the decision each time.
Every figure below is priced on the ladder we record in 0.5 kg steps at the 2026-09-25 review, with handling at £1.00 per parcel and £1.00 of extra handling for a merge (Recorded, latest of 12 reviews since 2026-04-02). Those two handling lines appear in every comparison that follows.
1. Write the chargeable weight of each parcel as it stands today. 2. Measure the single box you would actually use for both. 3. Price all three figures on the same rung set. 4. Count how many claims you would hold afterwards.
Case one was two hoodies at 1.0 kg chargeable each. Case two was a 2.5 kg parcel and a 1.5 kg parcel, the pair that lost money. Case three was a 3.0 kg parcel and a 2.5 kg parcel, which saved freight and produced the write-off.
The three cases were chosen because everything except the parcels stayed the same: one warehouse, one pricing ladder reviewed on 2026-09-25, one handling fee of £1.00 per parcel and one £1.00 merge surcharge. Case one shipped 1.8 kg of clothing, case two 4.0 kg of bulky goods and case three 5.5 kg across two similar parcels, so the differences in outcome come from geometry rather than from anything seasonal.
Pricing the three cases rung by rung makes the pattern visible. The wins sat at 2.0 kg and 5.0 kg, where the merged weight stayed within half a kilogram of the sum of the parts, while the loss sat at 7.0 kg against parts of 2.5 kg and 1.5 kg, where the single box added 2.7 kg of volumetric weight that neither parcel had carried on its own.
What the merge actually saved
Two hoodies kept apart cost £8.20 plus £1.00 handling each, which is £18.40 for the pair. Merged into one box they billed at 2.0 kg chargeable, which is £11.60 plus £1.00 handling and £1.00 extra handling, so £13.60 in total (Recorded, 3 merges).
The £4.80 came from structure rather than from weight. Two parcels pay the opening rung twice and one parcel pays it once, and that duplicated rung is the whole saving on soft goods that fold flat and add almost no volume when they share a box.
Case three saved £6.10 by the same mechanism at a heavier weight. A 3.0 kg parcel at £16.00 and a 2.5 kg parcel at £14.30 became £24.20 together, because the merged parcel billed at 5.0 kg rather than carrying two opening rungs and two handling fees.
Set the saving against the goods and the pattern holds. Merging pays when the combined weight stays close to the sum of the parts and the volume does not grow, which is true of clothing and false of anything rigid, boxed or moulded into a shape.
Handling is the quiet half of the saving. Two parcels cost £2.00 of handling and one merged parcel costs £2.00 as well, because the merge surcharge cancels the fee that disappears, so the £4.80 in case one came from the duplicated opening rung and not from the handling lines that look larger on a screen.
What it cost us in flexibility
Case two lost £7.40. A 2.5 kg parcel at £14.30 and a 1.5 kg parcel at £10.90 cost £25.20 apart, while the merged box measured 45 by 35 by 25 cm and billed at 7.0 kg chargeable (Recorded, 3 merges).
At the 6000 divisor that box is 39,375 cubic centimetres, or 6.56 kg of volumetric weight, which rounds to 7.0 kg. The merged parcel cost £20.10 plus £4.20 for the weight above the 4.5 kg boundary plus £2.00 of handling: £32.60 and a loss of £7.40.
Case three cost more than freight. The parcel carried one claim because it had been merged, so when two items arrived damaged only one could be put forward, and £34.20 was written off against a freight saving of £6.10 on the same box.
1. Photograph every item before the merge. 2. Note the value of each one separately. 3. Check whether one claim would cover the most valuable item in the box. 4. Refuse the merge when it would not.
Flexibility has three parts: splitting a parcel later, returning one item without touching the other, and holding cover per item. A merge removes the first two outright and weakens the third, because one claim has to serve everything in the box, and that is a real cost even when the freight figure improves.
Waiting for the second parcel also has a price in days. Case three needed four extra shelf days for the second parcel to arrive, which cost nothing inside the 30-day free window and would have started a storage line had the wait run to five weeks instead of four days (Recorded, reviewed 2026-09-04).
The third flexibility cost is the one nobody plans for: a merged parcel cannot be returned in part. Returning one item from a merged box means returning everything in it or filing a claim for part of it, and on a parcel holding a £34.20 item and a cheap one the choice is between losing the cheap item and losing the claim.
Where the maths would have flipped
Two thresholds decide most merges. The first is the 0.5 kg billing step, which makes a combined weight of 4.4 kg cheaper than 4.6 kg by a whole rung, and the second is the 4.5 kg boundary where fixed rungs give way to £4.20 for each extra kilogram.
The third variable is the divisor. The same 45 by 35 by 25 cm box priced with a 5000 divisor would have billed at 7.9 kg rather than 6.56 kg, so 8.0 kg after rounding and another £4.20 on the same parcel of goods (Recorded, 2026-09-25 review).
Case two would have broken even at about 5.5 kg chargeable rather than 7.0 kg, which is roughly 1.5 kg of volumetric weight or 9,000 cubic centimetres of box. Repacking the two items into a 45 by 35 by 20 cm box would have crossed that line and turned a £7.40 loss into a small saving.
1. Price the merged parcel at the weight you measure. 2. Price it again half a kilogram lighter. 3. Find the weight at which the merge stops losing money. 4. Compare that figure with the box you can realistically pack.
Case one would have flipped at a combined weight of about 3.5 kg. Below that figure the merged parcel stayed on the same rung as the sum of its parts, while the £4.80 saving disappeared once the single box crossed into the 4.0 kg rung, which is 9,000 cubic centimetres of extra volume on a clothing parcel that should compress easily.
Turn the three cases into three branches before the next decision. Merge when the combined weight sits within half a kilogram of the parts and every item is under £25.00. Split when any item is rigid or boxed. Hold the second parcel unmerged for a week when the only reason to merge is a rung you are not sure about, and price both arrangements on the day you decide.
What we would do again
Merge soft goods and split anything rigid. Clothing compresses inside a larger box, while boots, boxed items and anything moulded dictate the dimensions of whatever contains them, and that difference alone decides the volumetric figure on the label.
Check the rung before checking the price. A combined weight of 4.4 kg sits on a cheaper rung than 4.6 kg, and 200 g of packaging can move a parcel across that line on its own without a single item inside it changing.
Keep the claim count visible while the decision is being made. One claim per parcel is the rule we learned at a cost of £34.20, and a merge of two items with different values is exactly the case where a small freight saving gives up the most protection.
1. Measure both parcels as they are packed now. 2. Measure the single box you would use instead. 3. Convert both to chargeable weight at the 6000 divisor. 4. Merge only when the saving survives the claim count.
The decision now takes two minutes and three numbers: the chargeable weight of each parcel, the chargeable weight of the box, and the value of the most expensive item going into it. Where the first two are close and the third is under £25.00, we merge without hesitating, and where any of the three fails we ship separately.
One comparison closes the loop across the three cases. The two merges that saved money both kept the combined weight within half a kilogram of the sum of the parts, while the one that lost added 2.7 kg of volumetric weight, and that single figure separates a good merge from a bad one more reliably than the freight total does.
The data point behind this note
Merging two 1.0 kg chargeable parcels cut the recorded freight from £18.40 to £13.60, a saving of £4.80, while one bulky merge raised it from £25.20 to £32.60 (Recorded, 3 merges).
Rates last checked 2026-10-02. Where a figure is community-reported we say so; where we could not verify it, we write Not verified instead of estimating.