If the following appears:
Payment error: (invalid address or customer)
Go to Modules and click on Configure where it says ePay
Chance the setting for URL-decode cookie in relay-script to the opposite and click Update settings
The ePay payment module for PrestaShop supports the One Page Checkout module from Canin
The One Page Checkout module is supported from PrestaShop version 1.3.1 - version 1.4.x.
Be sure to use the latest version of the module.