I suggest using a card clip
For buckled decks, farro or riffle shuffle them half face up into half face down. Put them in the truck case (if you're missing some cards, you may need to fill the truck case up), put that in the card clip so its snug. Leave it for a few days. If you don't have a card clip and don't wanna buy one, put the case on a flat surface, put another flat surface on top (like a hardcover book) and stack some things on top of the flat surface for weight (you can do this with the card clip too).
After a few days, take them out of the clip and just shuffle them a bit to break them in again (you don't need to break them in like a new pack, just get them loose again).
You can even use the cardclip to carry the deck around to keep it from getting beat up, or store your decks in them to prevent future buckling.