I would ask this question in the X-Cart community forums or in their support desk. The key phrases you would want to use are "API" or "user exit" - in essence you're looking for something like the PayPal Instant Payment Notification - an application script that would get posted with the purchase information each time a purchase was made.
I've done this with the PayPal IPN - we notify the warehouse with shipping information at the time each order is received. It has worked flawlessly without human intervention for over 4 years. So you're on the right track!
best regards, ~Ray