There a number of really good cost savers out there if you're shipping USPS. Due to the breadth of with USPS you can save money by simply adjusting the classification of your packages. The simplest change you can make is from Parcels to Flats.
Flats are classified as having one dimension greater than 6-1/8 inches high OR 11-1/2 inches long OR 1/4 inch thick, and no more than 12 inches high X 15 inches long X 3/4 inch thick. That change alone can save as much as 45% on lighter packages.
Here's a breakdown of the cost difference:
Next up is Media Mail. Are you shipping books, CDs, or other media? If so, Media Mail is a huge savings -- bigger than that of Parcels to Flats -- over standard First Class shipments. It also goes above the 13 oz cutoff for First Class (to Priority). Here's a link to the guidelines for what qualifies as Media Mail:
For those of you doing large amounts of shipments that aren't time sensitive (or very expensive), Last Mile is a great option. It's a blended service between the USPS and FedEx, UPS, or others. You'll sign up for the account with the carrier doing the pickup (FedEx, UPS, etc), and they'll inject the packages into the USPS mail system.
One easy way to save money on shipping is to lighten your load. Are you shipping heavy items? Or using heavy packing items? By shipping your items as light as possible you can save a lot of money on shipping.
What time does your local Post Office or carrier store close? What time does does your carrier pickup? Knowing these cutoff times can drastically improve your shipping times. If you make sure to always have orders ready to go by then, you can save yourself an entire day by not missing the cutoff.
Even better, let your customer know the cutoff times. Post sometime like this throughout the shopping cart and checkout pages: "All orders shipped before 3pm will be shipped the same day!"
This will drastically improve conversions and satisfaction. The number one reason people choose Amazon is reliability around shipping. They know even before the item is in the cart when it will arrive by. This is a piece of that and something you can easily do.