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, adn check that sufficient funds are available.
These are classes that are used to calculate the cost of shipping items, they may act locally, or they may connect to service providers (like ups or usps) to calcuate shipping prices.
Order Total Modules
Order total modules are used to calculate/display sub-totals, tax, totals, and other intermediary values for an order. Special order total module, that provide for calculating discounts. or special charges are slso the province of the oder total modules.