AWS Marketplace Pricing Model -- Flat fee

I am a seller on AWS Marketplace and I am looking to set up a pricing model where buyers pay a flat fee to use the product and charge them per request they make to the API. For example, pay 50 dollars (base price) to use the service + usage fee (0.01 per request). I have seen the SaaS contract + pay-as-you-go model and I am not sure if that would suffice, it seems like the base price would have to be the same consumption unit along with additional usage.

For example, pay 50 dollars ( for 500 requests) and then pay + usage fee (0.01 per ADDITIONAL request) which is not what I want. I am just looking for the buyer to pay a monthly initial cost for access to the API.