Shipping advice -- avoid postage due!

I've had a couple of trades here, and a couple of orders over on another site, in which I ended up holding the bag for a big postage-due bill. The most recent was $4.67! The other trader (in their innocence, being new to this) shipped about a dozen cards in a stack, protected by corregated (thick) cardboard on either side, and they folded over a long-size white envelope to make a tidy little package. However, that WON'T BE ACCEPTED as first class mail. So a "Forever" stamp won't cut it. I got billed for the extra postage as though the shipment was a first-class package.

Here's a tip I learned recently (from Reddit, I think): take one of your nine-pocket trade binder pages, and cut one row of three pockets off (horizontally). Put your cards in the pockets, evenly distributed. This will now fit into a long-size envelope. However, it still won't ship for a single Forever stamp--you should add an "Additional Ounce" stamp, or possible swap out for a "Non-machineable" stamp. Both options are much less expensive than shipping something in a padded envelope, even when using a service like PirateShip (highly recommended, BTW).