I had that problem last night. I clicked on the mailbox, and it said get mail. So my sim collected it, there was no pop-up saying what it was. Now it's stuck in his inventory. It looks like a letter. If I click on it, no description comes up. I can't drag it out to place it anywhere, and I can't drag it to "sell". I don't even know what it is. It's not a bill as they paid bills only a couple of days previously.
EDIT - Just googled it. Apparently if you pay a bill by phone, the physical bill is still in the mailbox. Apparently it disappears once you get another bill.
