People are enrolled manually OR by PayPal, never both. If someone is manually enrolled, he/she is not prompted to pay because he/she is already enrolled.
Imagine you have 3 payment options available, if someone pays using PayPal, he/she should not be prompted again to pay using Authorize.net or Pagseguro. Once enrolled, a participant is never asked for money again.
If you want payment on module or section level, you can have a look at the availability PayPal.