It's what the supermarkets and delis use to cover up their spreads after they close. It's a sheet of cellophane that covers cream cheese, butter, peanut butter, egg or tuna salad, lox etc.
A spreadsheet is an old bed sheet that I cover my dogs crate with. It gives them a sense of being in a den.
This is a painful thread to read. I've had three packages stolen in the last 30 days. All were eBay purchases and all were numismatic, although not nearly the value of the OP's notes... Each package was marked as having arrived at the local Post Office and was out for delivery then they all vanished... So, definitely someone in the Postal System with sticky fingers. I've been in touch with the Postal Inspection Service...
Sorry to hear that but contacting the inspection service is the right step. I hope they catch the thief and recover your items.
I should add to try using UPS. It cost a little bit more but it’s worth it. I’m retired postal and I ship UPS.