It is quite rare that you'll need to void a transaction. There are two ways you can handle a void. You could clone CreditCard.pl and modify the second version to only process voids. Alternatively, you could request that Magma process the void manually for you. If a transaction is submitted and one or more days have passed, and you want to reverse the transaction, you'll need to submit a 'return'.
Returns are dangerous.
In effect, you as a merchant are giving money away. Because of the danger, Magma does not generally configure it's merchants' accounts to support 'return' transactions over the Internet. If you need to process a return, simply FAX Magma your request with all account details, and Magma will process the return manually. Security Magma's E-Commerce service is extremely secure. The connection between your customer and Magma's server is encrypted using the SSL security protocol.