Just to clarify a couple of points, are you using Power store 2?
if so, Authorize.net and PayPal Express Checkout are already implemented, you do not need to use eCart to add these payment gateways.
Paypal Payments standard is not supported in PowerStore, you will lose functionality if you try to switch to payments standard.
To use multiple shipping providers, you would need to create a select list on the checkout page for the user to chose. On the confirm page, you would need to store the chosen selection in a session variable.
After adding the server behaviors for the shipping providers you want to use, you will need to hand code the trigger for the server behavior to compare the session variable value so the look up only ocurs if that was the chossen shipper.
You would also need to edit the shipping rule in the eCart objest to add the session variable value as a trigger. We do not have any explicit documentation for this, but it should be pretty strait forward if you are comfortable writing PHP code, and using eCart.
