Sounds like the require_once command at line 13 was coded as:
require_once("/WA_eCart/Adv_CO_Scripts/PP_DirectPayment_PHP.php")
try removing the leading forward slash:
require_once("WA_eCart/Adv_CO_Scripts/PP_DirectPayment_PHP.php")
Most likely it is being added as a result of the "Create Links relative To" setting on the Local Info tab of your site definition. Set that option to Document instead of Site.