RSVPMaker has supported integration with PayPal for years, but the original integration method it used is now outdated. The plugin has now been updated to include support for PayPal's REST API, which makes allows you to display PayPal and credit card buttons directly in the event registration confirmation message.
If you run a website that currently uses the older PayPal integration, I recommend you update your RSVPMaker settings using the REST API credentials (which are different). Here's how to set that up: Tutorial on PayPal Integration (REST API).
I also prepared a quick video demo: