Actually the shipper is NOT the USPS it's DHL who subcontracts to the USPS to consolidate packages for cost reduction. This is why packages shipped via DHL often have weird tracking steps, your tiny package is being moved around to fill empty space in transportation vehicles to keep fuel and labor charges at a minimum. If you have any doubt that the cost reduction works, look to the fact that FEDEX & UPS do the same thing and all four organizations USPS, DHL, FEDEX & UPS are showing lower costs and higher profits for the packages done this way. (the trade off for higher profit is longer in transit time and weird tracking timelines)
BTW - FEDEX is no stranger to this concept it's what their business was built on. If you're old enough to remember their founding you may remember their business plan, the fastest and cheapest method to ship overnight from Boston MA to Springfield MA is via Nashville TN.