How to Get SamCart Subscription payments onto Keap

Hi. I’m trying to get my samCart subscription payments onto Keap Max Classic.
The initial order is integrated no problem at all. But the subsequent subscription payments do not get added, these have to be done as a separate integration.
I have my product mapping set up, and I have the subscription payment integration set up. But SamCart will only send the subscription data over when it finds the corresponding Product ID and Contact ID in Keap along with an unpaid invoice, that it then marks as paid.
So I somehow need to create an unpaid invoice for the subscription in Keap, each time a subscription is charged in SamCart so it can match them together.