Developers - Modules
Modules are php classes that can be 'dropped' into Zen Cart to provide extra functionality during Checkout.
Currently there are 3 types of modules.
These are classes that allow you to add different methods of collecting and validating payment details. This could be something as basic as allowing a COD payment method where no details need to be checked, to connecting to an Authorize.net account to validate the credit card details and check that sufficient funds are available.
Order Total Modules
Order total modules are used to calculate/display sub-totals, taxes, totals and other intermediary values for an order. Special order total modules that provide for calculating discounts or special charges are also the province of the order total modules.