of course they can open it up, it's the government and about the peanut butter thing, i dont think it's that weird, just say the person getting the butter loves that brand and cant get it where they live, lots of ppl do it with jam, tea etc.
From one who knows: Bag it then take it to a room where you don't toke. Bag it a couple more times and dip it in melted parifin wax until thouroughky coated. Box it and mail it to your address, but not your name. WHATEVER YOU DO, INCLUDE A RETURN ADDRESS. No return address is the most suspicious thing about a package. Just make one up. It's outgoing mail- nobody looks at the return address unless it's being returned. And FYI- about 5% of the US incoming international mail is "checked". Which means a dog is walked by the bins of letters and packages. And no- the US gov cannot open your mail without a warrant or dog alert, unless you declare the package an agricultural product.