Accepting Insurance as Partial Payment

The company I work for is now accepting vision insurance as payment for the contact lenses we sell. I plan to modify the site in two steps to speed up our acceptance of insurane information.

The first step is to introduce two fields into the checkout process on the payment screen. The two fields would accept an identification number, the second would accept the insured coverage amount.

What I would like to see happen, is once the patient has entered their coverage amount, the total that is charged to the patient's credit card ends up being the difference of the total and the coverage amount. So, the total is $150, coverage is $75, and the credit card is automatically charged the difference of $75. I shouldn't have a problem adding the fields or modifying the database, but I can't figure out where to get it to change the amount charged to the credit card. I was thinking that I could add a button that would "Update Total" once the coverage amount was entered. Any ideas?

The second step is much more involved. We are only accepting insurance from one insurance company, our parent company, so we have full access to their mainframe. All insurance data is actually on an AS/400. My plan is for the patient to be able to enter their identification number, hit a button, and the fields then are automatically populated by grabbing the data from the AS/400. From there, the the same concept as the first step above would take over.

My question on this is, does it sound feasible? Has anyone tried anything like this before?