WHMCS affiliate commission on billable items

Hello all we use whmcs and have an affiliate program. the commission process works fine on orders by assigning the % specified in Whmcs settings. the problem we have is that we add billable items each month (we run an api which collects information on clients usage of a voip service) but the commission for the affiliate is not added to their balance. We would like a script which runs daily to check a clients billable items (possible next invoice to be added values) and apply the commission to the affiliate. The commission we have is 10% but we can customise this in whmcs so would be preferred to look up the whmcs setting per client/affiliate. This is not too important though so just using the 10% will do. the commission for each billable item can only ever be added once. We have working live whmcs so would require the script for testing on our servers and would expect help in getting it working. To show that you have read this project in full please put the word "whmcs" in your response. thanks Dave