Java Point Of Sale + Scripting + huge mess

[lxbui]

Hi,

This question is about Point of Sale Applications. My friend owns a liquor store, and has asked me to do a program for him so that he can accept coupons that are loaded onto his customer loyalty cards. I'm running into some problems.

1. His POS system doesn't have a loyalty program built in, so I have to ghetto rig it by writing code.

My problem is where to start? Here's the normal scenario.

1. Customer scans a 40 oz. bottle.
2. POS system retrieves the UPC code input from either the scanner or the keyboard.
3. The POS system queries the database for the UPC code, then returns the price, description, etc.
4. The POS system calculates the total, taxes, etc. and the transaction moves forward.

Problem:

1. Customer scans a loyalty card.
2. Error - POS system queries the product table rather than the customer loyalty card table and returns a message.

Questions:

a) Without messing with the code on the software POS program, how do I get the Software program to read from the second data table?

b) Where do I begin? Where can I get additional info to read?

I have an idea on what to do, just don't know how to implement it. I think I need a script that passes control around in the main software POS program. I need the script to check the UPC to see if it's a UPC or a loyalty barcode. If it's a UPC, hand it back to the POS program. If it's a loyalty code, query the database and return the discount.

The problem lies in if it's a loyalty card, how to I pass it back to the main program when I can't access the source code?