As a letter or as a package.

Letter envelopes with letter postage are for paper only. You can do it and people do, but if you go to the counter they're not supposed to offer you this service because envelope rates are for paper only, unless you're netflix and work out some kind of special deal.

The only other option is package service that requires your 'package' which can be an envelope to be at least 3/4 inch thick. First class will have your flimsy envelope mingling in with packages that are up to 1lb (maybe heavier?) and parcel and priority will have your package mingling with and getting thrown around with 5-10-50lbs packages. You can literally throw a barcode and address on a tire or hubcap and throw it in that mail group.

Anyway, it's not just a bad idea, it's actually against regulations. Most postal workers don't know all the regulations. You only learn them by shipping thousands of packages, which almost none of them have done. Ive lost 1 out of 2000 packages (and 40% of sales are international), and it was a domestic package that didn't meet that 3/4 thickness guideline.

Anyway, he didn't know it was against postal regulations, just a very bad idea so he might have given a man a hard time about it too..