Processes a payment in the store for a given user's cart. This implementation
processes a request through PayPal Payflow Pro's ACH processing for
electronic payments. The process(Map) method manages the process of
sending a request to PayPal, and handling the response appropriately.
parameters - A Map containing the values of the user's
submitted payment information.
A Map containing a least one key named
"resultCode", which contains a String parsable into an int,
representing the results of the processing. 0 for the resultCode
indicates success. Optionally, the Map may include
additional keys, such as "resultMessage", a string describing the
results of the processing.