Processes a purchase order payment for the store. Simply records the fact
that the user elected to by pay with a purchase order, so that the order can
be looked up and processed after payment is received.
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.