Anyone who knows an app that integrates with payment gateways?

You need to be more specific

This should allow customers to pay their invoices in the Customer Center through payment gateways (Stripe, Paypal, Credit / debit card, etc)

Hi Lea, We are a Stripe Technical Partner and have a Stripe integration that is native to NetSuite (i.e. sits within your NS instance with no 3rd party software involved). Happy to send you some information if you would like? just pop me a message at charlie@suitefunction.com . Thanks