Haven't done it myself, but would one of those cardboard card storage boxes work? Maybe you could cover the inside with a soft padding like tissue paper or bubble wrap, Saran wrap 50-100 cards together, and lay them in end to end. Then fill in any gaps with more padding. Basically make sure the cards can't slide against each other (to prevent scratches), and that the edges can't get nicked.
Of course with the fit being so tight any damage to the box might end up damaging the cards. So as an extra precaution, I'd put that box in another one and again use padding / packing material (somewhat looser this time) to make sure the inside box doesn't bounce around too much. That way even if the outer box gets banged up the cards will still be safe.
Well, there's one idea.
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Speaking of packaging cards, I'll just throw this out there since I've gotten some cards damaged in the mail this way. I'd highly recommend that the openings of toploaders (and the sleeves sandwiched between them) be sealed, since cards can and do slide out, and in some cases get badly scuffed since there's only a plain envelope protecting them. On the flip side, using tape to seal the opening can result in cards being stuck to the tape. Generally I just fold a piece of paper over the opening and tape it to the toploader.
