There is an obscure USPS rule that says a piece of mail needs to be at least 3/4" thick in order to qualify for delivery confirmation. (so 99.9% of bubble mailers would not qualify). If a very finicky postal worker sees a bubble mailer with a prepaid label they are within their right to upgrade it to priority (or RTS it).
My old post woman used to give me SO much crap for this. I had to go to the town over to mail out my cards.
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