Some of the best packed stuff I've received was very simply packed.  Your choice of boxes is great - the smaller one will hold two decks at most.  A single deck is 2.5 x 3.5 x about 1 inches.  There's a clear plastic bubble "bag" I've seen people use for sending me stuff that's practically custom-tailored for decks - perfect size and has a sticky strip for sealing it shut.  Drop those in the box, use old newspaper to fill in the gaps and stop shifting, and your box should survive most typical mailing disasters.  Use clear packing tape and SEAL EVERY SINGLE EDGE WHERE THERE'S AN OPENING in order to the chance of anything getting snagged on your box and tearing it open.

If you're planning to ship more than just a few decks, better to go with a box and added padding.  Nothing will beat the protection a box will provide, but if you wrap your small deck shipments in cardboard before tossing them in the bubble mailer, you've essentially made a box in the bag, right?    Best thing to remember, whatever method you're using, is that nothing should move or shift around inside - the less it moves around, the better the survival chances.

And if you ship internationally, forget the mailers.  Gotta use a box.  International mail takes a serious beatdown, especially when using First Class or below.  Anything less than a box is a gamble.  Wrap it like you're building a fortress!