Does USPS offer small package service and pick-up for hundreds of parcels a day at a Summit or Jegs like UPS or FedEx? No way the vendors will box it up and then pay someone to drive over to the post office to stand in line and mail it.

At the same time, high-volume shippers do get shipping discounts and may not pass on the discount, so you may be paying list price while they make more GP$ on the freight out. You might look on the UPS site and see the walk-in retail rate and a shipper with a commercial account might be paying 1/2 or less.