When fulfilling orders to your customers, you'll obviously want to reduce your shipping costs. The big shipping carriers (USPS, FedEx, UPS) offer discounted rates for businesses who create a professional account. Usually the discounts are based on how many packages you ship over time. The more packages you consistently ship, the larger discount you'll receive. However, it's very difficult to become a large enough business to where the rates are truly great.
But there are services (Pirate Ship, Ship Station, Shippo) that have partnered up with the big shipping carriers to offer you those discounted rates. It's as if all us small businesses banded together and used the same professional account. This way we all get greater discounts.
I personally use Pirate Ship. It's a completely free service. Instead of printing labels on the USPS or UPS websites, you'll now simply print labels through the Pirate Ship website. And then you'll save a couple dollars on each package for no reason. It's awesome. Easiest money you'll ever save.
While purchasing USPS shipments through Pirate Ship, the mass commercial shipping option is known as Cubic. This is typically the best rate by far if you're shipping within a few hundred miles. Anything beyond that range and I switch to Flat Rate boxes.