Check out the various mailing houses, they have volume discount with USPS. If you have the volume and do not mind paying a yearly fee, you can sign up with USPS for volume discount too.
In our case, we mail out iPhone cases. If we mail through USPS at a retail location, they charge us $2.32 just for the shipping cost. Through a mailing house, we get the cost down to <$1 if we ship 500 pieces with them at a time. Half of this cost is for the mailing house to buy the envelop, print the label, package and sort the item, and the other half is USPS postage fee. So, in our case, it is a substantial saving over USPS retail. Your mileage may vary depending on your item size and weight.