[quote name='DustBoogie']Box. Small box. If you're offering Priority Mail, just use the small collapsable boxes that you can get for free at any post office.. GBA games and the like fit in there perfectly (with room for some padding).
Heck, if it's eBay, do it Priority no matter what, they're paying. For trades, I usually have a small box lying around to use to send them. Never send in a bubble mailer unless you flatten the box and attach it to a sheet of cardboard and the buyer is okay with it, and
NEVER send in a plain envelope.
You'll probably get less bitchy advice from other CAGs, but I am an absolute pain in the ass about my GBA boxes

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I've sent out a ton of complete GBA games on Ebay and used the advice up above, Priority Mail boxes with padding are the way to go.
Just put the game in the center of the box because sometimes the sides get a little beat up.
I just got some of the large video boxes from my post office yesterday, so they aren't being completely phased out yet... it takes way too long for the post office to get them to me when I order them, about 1 - 2 weeks minimum.