for sets/lots of 100-250 basic cards (or approximately 2/3 that many for chrome or even fewer for thicker stock items) i go with small flat rate boxes (5.15 if you print your mailing label online) .....2-4 stacks of cards in team bags , stacked in piles of equal height and then sandwiched between 2 rigid pieces of cardboard cut to fit snugly in the box (that leaves a cushion on all 4 sides so in theory no dinged corners or edges)....
larger lots i would agree with anglonomics and always double box...a medium flat rate box runs 10.85 when paid online (a little more than a regular priority box needed for 330 cards but a definite bargain when shipping larger lots - even moreso when you consider the extra protection double-boxing offers).....
if you really don't want to double box , then you'd need to pack the box VERY TIGHTLY (i've done this and even added decoy cards on the back , front , tops , and bottom to try to offer SOME cushion) but no matter how well they are packed to avoid movement , all it takes is having the package dropped on one edge or corner to ruin everything.....personally i'd only take this route for very low-end items that i could replace for virtually no $$$$
