Pre Sale question: concerns item_number

So here's my question.

I already have a paypal purchase / download button that I am using on my site, it works great but with one exception.

Its' download function is premised on receipt of a paypal return via the IPN Notify URl. Like most paypal related processing it then sends the IPN message back to paypal for validation. All good so far.

What I'm selling is a piece of software that I've invested literally years developing: evenings and weekends while working fulltime. As a result of the effort I've put into this thing I also contracted with a vendor of software licensing.

Both of these, the paypal related logic and the licensing software (a web service) rely on the content of the item_number field the message sent to and returned back (via the IPN message) from paypal.

My problem is that the item_number is defined within the paypal logic I've already installed and within the licensing web service. Each of these two assign the number sequentially as 'products' are defined to each.

In the situation with my existing paypal related logic the product has a very high item_number because I also have 90+ documents that I allow to be downloaded without a charge because these documents are used by my software to control itsoperation.

In the situaltion with the licensing web service the item_number defined there is very low because there is currently only 3 prducts defined, 2 demonstration products and my own real product.

I have a paypal listener developed that is the target of the paypal Notify URL where I 'catch' then broadcast the IPN message to both the download logic and to the licensing web service. Each of these two send the message back to paypal for validation.

Since teh number will never be the same (neither of the two products allow that number to be manipulated once established) I tried to alter the IPN message before it got sent back, but since each send the message back and paypal validates that against the original that it holds onto, one of the two (download or license) fails.

So ... does this product allow for the control of the item_number?? If I could assign it by some setting on the backend, then your product would solve my problem.

Thanks - MikeG