The Postal Rate Commission approved the U.S. Postal Service's proposal for a two-year experiment to test two flat-rate Priority Mail boxes. The rate for the boxes is $7.70, regardless of package weight and destination. The USPS chose the rate for customer convenience: Two Priority Mail $3.85 stamps will cover the postage exactly, with no need to look up zone charts or weigh the parcel. Consumers or businesses, however, can put any type of stamps or meter strips on the boxes.