MasterCard has an automatic rebate program called Easy Savings that works with “a MasterCard small business or select middle market card issued in the United States, from a participating bank.” This includes certain business debit Mastercards. You can find out if your card qualifies by visiting the website and typing in the card number and trying to register for free.
Here is a list of participating merchants. Two newly added offers are 5% back at the U.S. Postal Service Click-N-Ship website (online postage) and Dunkin Donuts shops. I know some active eBay sellers that wouldn’t mind a 5% discount on postage. The rebates are automatic and show up on your monthly statements. You don’t even have to click through a portal like eBates, just shop like normal.
My CitiBusiness Mastercard and WaMu Business Debit cards were accepted. Given that any individual (as a sole proprietorship) can get either a business credit card or business checking account (with debit card), this is then basically open to anyone who wants it badly enough.