I have had the post office tell me they could try and send a thin bubble mailer (with a top loader inside) as a letter since it fit through the slot in the template they use. But, the guys said he would be afraid they could decide at the receiving post office that they are going to charge it as a parcel and charge the recipient the extra postage. So, I just bite the bullet and do the $2.32 when I send bubble mailers.
For 1-3 cards with a value of $10 or less I do PWE non-machinable if agreed upon on both sides of the trade and put the cards in a (used) GU top loader.
I think that's why people like sites like COMC since you can buy a bunch of things, even submit counteroffers, then pay the $3 flat shipping for all the cards that you buy.