I know that I can’t do this directly, but is there any way that I could offer (for example) a pair of earrings which would cost £5 if sent in an organza bag or £10 if gift boxed? (Those aren’t real prices, but hopefully you get my drift). At the moment, I’m having to second guess whether an item is intended for the purchaser or as a gift. Could I have an additional listing for gift wrapping as an add-on at the appropriate price?
You could do a listing for it. Maybe call it ‘Gift Wrap upgrade’ with a picture of some gift wrap/gift box and a cost. In you other listings add that you do a gift wrap service and they need to add it to their order with a note for which item is to be gift wrapped if they buy a few things?
I’ve seen some shops have a collection dedicated to upgrades - extra gift wrapping or guarenteed next day delivery - that way the listings for those things don’t get lost in your shop. Check that your gift boxes don’t push you into the next Royal Mail size bracket or make sure the postage associated with the gift wrap covers the increase from large letter to small parcel.