Flate rate shipping
One addition that would make PowerStore much more flexible would be to add an option in the Shipper drop-down for Flat Rate. When selected, you only need one input box for the rate.
For my drop shipper store, I get a much better rate using his FedEx account than I could with my own account. I charge a flat rate of $2.99 for all purchases. Research shows that customers look at it as the same as free. I build the difference into the price for each item.
This keeps it simple, and removes dependencies on outside sites. Nothing more frustrating than having your site grind to a halt because someone else's site is having problems.