If you are using pre-paid satchels, I'd take a photo of it after it's addressed (I do this anyway, with the items being packed into the parcel, for a variety of reasons). The other thing you can do is request the destination postcode be entered when the packages are scanned in, as that will show up on the receipt and provide at least some additional, matching info (this may also help if whoever is scanning in is doing something incorrectly. Occasionally the safff at my PO will miss something and a package won't appear on the receipt, but they know I want the lodgement receipt for all packages so if they spot it themselves they'll re-do it).

I create my labels through My Post, though, and also save a copy of the PDF that's generated, which contains all of the delivery information so can prove that way the exact address the package was sent to if it's ever needed, though you do need the delivery scan for eBay as well. .