Do you mean that you have listed a few items on separate listings, each with their own postage?
Or that you have more than one of a specific item and have listed with the quantity available, in which case the buy clicks how many they would like?
I list separate items and will often add in the description that I will give a discount for combined postage.
Recently I’ve been selling a collection of magazines and listed each one seperately with the postage for one edition, and in the description have given a list of the combined prices depending on the weight, eg one for £, 2 to 3 etc for £ and ‘up to 12’ for £
If a buyer takes multiple copies I then ‘send an invoice’ which covers all of their items and I enter the reduction either as a discount, or changing the postage to their options of Royal Mail / couriers etc at the combined prices.
The buyer then pays for all in one go.
Sometimes a buyer has paid for the first one with single postage, but also has a bid on another that hasn’t ended yet - I offer them to send now or hold pending the outcome of the other bid
(Some say to send now, others ask to wait)
If they get out bid then I send the first as it’s already paid for, if they win the second then I send an invoice for that item with a discount (and a remark to say that it’s for combined postage)
In the first version, if it’s paid in one go when I generate the label the tracking shows for the whole set, in the second version if paid separately then the tracking only automatically shows on one item, so I add the tracking number to the other