thats always fun when that happens, especially when you use there APCs.
Your best bet as a seller is that if its an item you want to make sure you have a record that it went into there system correctly, have them scan the item and get an "acceptance" receipt. Those I haven't had an issue with having show up in the system. In general, as if for some reason an item does get lost and you don't have someone showing it in there system, it is quite possible they would just say you never really shipped it. So best bet, get the postage on it, and get it scanned in.