I wonder what kind of plugins can be installed to combine RoR and online
payment methods such as using Credit Cards or Paypal, etc?
Or must we create our own way to do the online payment?
The best way is to redirect your users to a service, such as PlugNPay, and let them log in there. That way, PNP manages their account, provides a call center, etc. You really don't want to do all that.
Then each time the user requests your premium services, you contact PNP as a webservice to see if their account is up to date. If it is, you log the user into their features, and PNP pays you.
No moving parts. Almost no programming. And no endless customer support problems.