As an alternative you can create a separate/new listing which is just for the postage only. You can then send the link to the buyer.

How much have you charged the buyer for postage? Hopefully you are sending as tracked. If it's the magazine you sold, I think it goes in the large letter registered envelope for just under $30.

Just as an aside, France is one of those dubious countries in regards to postage. Lots of issues and items that go "missing". Often delays at Customs and within the postal system itself, so buyers are quick to open cases.