As far as I know, all hosting services, has hiddn cost.
1. You must pay for set up fee (xxx),
2. you must pay monthly (xxx) even you have only one call per day.
3. there are some charges per call
4. something else, you will find when you get offer of invoice
All these (xxx) means 3 digit USD charge. You may end with up to 1000 USD/month, and do not have your own billing solution or you will go for solution 2500++ USD and have your sistem.
My calculations showing me that is better to invest in own Billing then spend same amount in 2-3 months and still do not have it.