I know that back in the day, EQ2 had an in-game ordering thing that allowed it’s users to use their EQ2 payment method to order a pizza. How would one go about doing something similar? Any ideas / thoughts on the matter? Perhaps Epic should reach out to these major companies and request for them to create a plugin for the marketplace 
You could check if they have exposed any api themselves, like simple http/rest requests (for which you already have the “tools” in the Engine)
Oh man, I remember that feature. If you remember how it worked back then, basically it would just open a browser window to the pizza ordering form. So basically all you have to do is use the “Launch URL” blueprint node/function. 
Good luck!
EDIT: Yep, I just tried it myself and it works.
API docs for it: FGenericPlatformProcess::LaunchURL | Unreal Engine Documentation
Still not 100% on how you would send the funds over, but that right there is step 1.