The coming rate hikes at UPS (NYSE: UPS  ) and FedEx (NYSE: FDX  ) represent a threat to the free shipping policies we’ve seen retailers expand exponentially, but a rate cut the U.S. Postal Service just won might give it a competitive advantage the delivery giants won’t be able to overcome.

Shipping stuff in bigger boxes is taking up valuable real estate inside delivery trucks.

In May FedEx announced it will begin using dimensional weight pricing to calculate rates on packages less than three cubic feet, and a month later UPS followed suit. Because both bricks-and-mortar retailers and their online brethren are increasingly shipping items that take up more real estate inside their delivery trucks, the carriers will start charging not only for how much a package weighs, but also for its size relative to its weight.

Dimensional weight pricing isn’t really new as both have used it for ground delivery services, as have air freighters and truck carriers, and the post office even adopted it a few years ago in what it called “Shape-Based Pricing.”

UPS and FedEx Rate Hikes Threaten Free Shipping and Lost Business.