Paypal billing question

[thenetbox]

Hi,

I want to make something for VB that would keep a running tab for a user. Example:

If user does an action the script would add $X into a database field. If he/she does the same action again then $X would be added to the previous balance.

At the end of each month I would like to either PM the user OR send a bill/invoice to the person with a paypal link that would charge for $X (total).

So does any one know how to send a Paypal bill for $X to the user? Thanks. (maybe I should have asked this on paypal forums :O )