to set up shipping in power store, go to the power store admin panel and on the settings tab select shipping.
if you wish to use USP to calculate the shipping: when creating the shipping rule, set the Provider to USPS and enter the details for your USPS account.
the base rate is used to set a flat rate for shipping, the per item create is used to set a flat cost for each additional item, if you enter values here, they will be charged on top of the USPS rate that is returned, if you don't want that, level them blank.
With express checkout, the user is directed to paypal to enter shipping and billing information.
since the shipping information is not gathered until the user fills that information in the paypal site, it is impossible to calculate the shipping rate until they return to the power store site.
I'm not sure where you where finding the documentation that you refer to but the built in help for shipping settings seams to be correct on the power store support page: