I believe Quickbooks can do everything for you. Not 100% sure, but I would look into that.
There are tons of Free QR code creators (or low cost). You could create a coupon, assign unique QR codes for each affiliate (place them on cars), and scan them in store to give proper credit to the affiliates. The only problem I see with this is: human nature. We forget things, lose things, and...
10% of the total monthly sales churning on an absolute basis is near fatal. That means that within 5 months, you have 50% absolute churn per year, which reveals fundamental flaws with the service itself. Anything above small single digit churn is telling you and your team that customers are not...