snd and rcv xml data

Have a customer asking to place orders via xml files rather than the user entry 
cgi web screens running on our iSeries using Apache http server. The web 
screens accept multiple pages of data, then generate an order#. They need that 
order number back. From many years ago, we also have ftp server receiving 
orders and creating return files with order numbers they can get. But this cust 
asking why they can't talk directly thru our http server, place the order and 
get a response. I assume I need a cgi pgm that will authenticate, rcv, calc, 
respond, but trying to understand how to format a response. At V5r3 and not 
(yet) running websphere server. Perhaps a few hundred orders a day like this..

tia
Jim Franz