Yes. Something like x dollars per customer per year would be slightly less optimised but more robust and defensible.
Don't forget that by providing you with sales / revenue / customer management they are bringing a lot of value to the table.
Who will develop the product ongoingly? Who will provide patches and support? These might seem like obvious questions to you but they must be discussed and written in a contract.
Finally, make sure your contract includes sufficient legal protection. i.e. the product has no guarantees and you are not liable for any cost, damages or loss, direct or indirect. If things go ahead then you definitely need a lawyer to help you with this.