Enter both, on every item. A courier charges for the greater of what a parcel weighs and what its size says it should weigh — so a missing weight is not a blank, it is a guess that gets corrected later at your expense.
The sum a courier does
Length times width times height in centimetres, divided by 5000, gives a weight in kilograms. You are billed the greater of that and the real weight.
A 40 × 30 × 25 cm box works out at 6 kg whatever is in it. If the contents weigh 1.5 kg, you pay for 6. What is chargeable weight?
What to enter
The packed parcel, not the item. A 20 cm vase in a 45 cm box is a 45 cm parcel. This is the single commonest mistake.
Weight including the packaging. Box, filler and tape are a real share of a small parcel.
Measured, not estimated. Kitchen scales and a tape measure are the whole toolkit.
What happens if you leave it out or guess low
The courier weighs and measures it anyway. The difference comes back as a charge on your balance after the fact — shown as a correction, long after you decided the item was profitable.
Guessing low does not save money. It defers it and makes it invisible. How to read your payout statement
Why it is worth doing in bulk
It is tedious per item and cheap in a spreadsheet. A bulk import takes weight, length, width and height as columns — so the time to do it is when you are listing fifty things, not when you are packing one. Listing a lot of items quickly
What it buys you
Delivery prices that cover the postage. The whole point.
An honest quote for the buyer at checkout, which is also what stops the "why is delivery so much?" message.
The ability to see which stock is unprofitable to post. A bulky item with thin margin is a decision, and you cannot make it without the numbers.
The items that catch people
Anything light and large: lampshades, cushions, boxed toys, framed prints, shoes in their original box. The scale says 600 grams and the courier says 5 kg, and both are right. Why does a big light box cost so much to send?
If you sell those, measure a packed box before you set a rate, not after your first sale.
If an item really cannot be posted economically
Offer pickup on that item instead. Delivery is set per listing, so one awkward thing does not have to distort your whole range. Giving each item its own delivery rules
More: how do I work out what to charge for postage, what is chargeable weight, how to pack an order.