Well if that envelope is more than 3/4inch thick or has more than 1/4in variation or more than 2 inches of movement inside the envelope or cannot bend at least 1 in (because it contains a rigid object for example)
Then guess what? It qualifies as a package and not an envelope and you must pay the same price as a box
If they're sending lots of these it can easily be worked-around. But yes, US postage is ridiculous. I've fulfilled packages that weren't time-sensitive directly from Hong Kong to the US as amazingly it was cheaper than sending domestically from a US warehouse.
I can send a 100g "large letter" from the UK to US for about £3 (about $5), and the only size restriction is that it must fit through a 2.5cm by 25cm slot, and can't be longer than 35cm. No bending or shape restrictions. For an extra £1 I can send a small parcel, which has even looser restrictions. The main problem is that over 2kg the prices jump massively.
Then guess what? It qualifies as a package and not an envelope and you must pay the same price as a box
So again.. $20