PowerStore is set up to work with Paypal Payments pro and Authorize.Net payment gateways.
If you own eCart, you can use eCart to change the checkout to use one of the other Local Checkout payment gateways that are included in eCart:
# Local payment gateway support
* Authorize.Net AIM
* LinkPoint API
* PayPal Website Payments Pro
* PayFlow Pro (PHP only)
If you wish to use a gateway not supported by eCart, you would need to hand code the logic to process the transaction with that gateway yourself.