Basically any quantity of mailings that meet the normal USPS bulk mailing requirements and include a QR code will qualify for a 3% postage discount.

The code must be marketing and advertising related and be for the product or service being promoted in the mailing. First class, standard class and non-profit mail can qualify.

Here’s a good FAQ guide if you want the specifics.

Offer valid for all mailings before August 31st, 2011.