You will find a number of third party services that will offer you discounted USPS shipping including stamps.com, shippingeasy.com, shipstation.com, etc. Note that many of these services are getting bought up by stamps.com, which actually may end up benefiting everyone as they will have more leverage to get better rates from USPS.

You'll also be able to get a discount when you use some shopping carts like Shopify.com.

Bottom line - shop around and see which one will give you the best rates - the higher the volume, the better the rate!