Yes. It’s a theme component. (At least most plans allow you to install whatever theme components you want). Did you try clicking the big blue button that says “Install this theme component”?
That sounds like a fair criticism!
I added a link to Introducing Discourse developer toolbar so you can find where the plugin outlets are.
I even added that link before I read this part of your message! 
Oh, but looking at the plugin outlets on If you want to stick text above the “Make a payment” heading, you’re in luck!/s it doesn’t look like there are any plugin outlets.
(Pretend “Pay for Services” is “Make a Payment”) You can put it either at top-notices or above-main-container, but I’m afraid that will add the stuff on every page, not just the payment page unless you can figure out some CSS to hide it on all of the other pages. If you’re patient, there’s a reasonable chance you can get https://ask.discourse.com/ to tell you how to do that.
OTOH, it would be a fair request to have a plugin outlet added on that page, but that’s another thing to learn.