I came across few sites providing lower rates for USPS shipping than USPS. As shipping cost is a real concern, where can I find more info on commercial rates? As we are selling to residential, USPS seems the cheapest. Weight of the boxes range from 12 oz to 2 lb.
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!