Personally I never use a customs sticker for lettermail (to USA and over 50 other countries).
The Green CN22 is NOT obsolete no matter what any postal clerk tells you. They are to be used for lettermail/letterpost
I've sent thousands of packages and lettermail (10k+) and all have arrived with the exception of 5 (Germany, Brazil, France & USA (2)). It is generally the exception rather than the rule . I would ask a buyer who has not received an item to be patient for up to 30 days after that I would be inclined to refund ragardless of payment method. If it was PayPal of course you might as well as you will lose any claim.

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915
- answering Trolls is voluntary, my policy is not to participate.