They linked these "mailjacket" envelopes in one of their notifications regarding this change.
They seem to fit the requirements I read so hopefully they work as I bought 200 of them and am sending out my first one today.
https://www.ebay.com/itm/6-x-8-Paper...53.m2749.l2648
I used that for a lot of two cards that was purchased for $12 total. For ones under $5 I went with a standard business envelope - card in softsleeve, card saver and team bag taped to printed off invoice.
Similar process that I have used probably thousands on times on Sportlots with very few issues.
Hopefully this all goes over well however I have seen some posts on Facebook thrifting groups and such where people are bragging about being pretty careless in their shipping including being able to put 8-10 cards in a card saver and mail that way.