Ask HN: I have users, I have traction, How do I get paid?

Hi HN, first let me say thanks in advance for your suggestions and insight.

I was hoping to get some advice on a side project I'm looking to monazite. I've built a social tool that is starting to pickup traction, and I'm looking at building a "pro" version which users would have to pay a monthly fee to use (it's a service). It would be great if they could just fill out their payment info and a few other pieces of data we would need, and then be on their way.

I'm not expecting a ton of monthly subscribers at first, and I'm trying to figure out the best way to charge users. Essentially I'd like to test the market without a huge investment.

I'm looking for the lowest barrier of entry. I've been considering using Paypal as it is incredibly easy to setup, and I won't have to mess with a merchant account. However, I've seen people completely bash Paypal around HN, and I'm not entirely sure why. Is there a better tool I should be using here?

Thanks for your advice.