What is the best way to ship 1000+ cards as cheap as possible? I have a bunch of commons of very small value and want to get rid of them while filling my pocket with a little more spending money.

USPS is your best bet, but you're still looking to pay $10+ ish, as long as it's not international.

USPS Flat rate boxes. The small one is $5-$6 and includes tracking. It can easily fit 100 unsleeved cards