Here is how I did. I have published the service I built on HN & PH. It was only a demo page that did not need signup to try. I had the pricing section as well, but when the user clicks the "Pay" button - I displayed a pop-up "Contact <email> for the pricing. As an early user, we have a discount for you". I manually created and sent a PayPal invoice in response to the emails.

We integrated the PG only after 3 months when we received at least one email per day for an entire week. In between, we were quickly patching the servers when needed, listening to the paid customers and building features and making improvements.