Sure, that's where you're at. You don't have to "assume" anything, your invoice is the paper trail the letter and its contents were sent by you and received at the grading service. That's good news, as it narrows down the suspects, by ruling out a third party. Now it's either you, or a postal employee. Just hang onto that invoice, and only give out copies, when necessary. That's your acknowledgment by an unbiased party there was indeed a $1000 note in the envelope when they received it. Then just cooperate with any reasonable investigation, and if the postal and insurance dicks do their job, don’t be surprised if you get paid. If things get out of hand, unreasonable, because this isn't a perfect world, that’s when you either be content to have learned an expensive lesson, or get your lawyer on the job. This isn't lost mail. Get the claim in now before the trail goes cold...
Ahhh I see (re:21 days). Ohh and you mentioned "...its contents were sent by you and received at the grading service." Note that this is just a purchase between private parties, no grading service involved. I bought it from a seller. He sent it (in theory at least) and I got an envelope. Just to clear that up. I'll gather the info and file ASAP. ...and then insert all the tar back in through their nostrils.