Edward from MyCryptoCheckout here.
Coinpayments offers same or even better service
What do you mean by "better service"?
big numbers
How many transactions per month were you thinking would be a good and fair number for both us and our users?
Or maybe to try first few months with an unlimited number of transaction monthly. Then you can get much more new users who will try your plugin(he'll probably help you to improve them if there any bugs)
"better service" means "checked and safe", also I would always take x% transaction fee than xx% yearly.