First of all, dummy APIs. I wish all the devs working on those a great life.
Now, I've tried using STRIPE even in 2019 but that was way above my league back then. Having to go from frontend to backend and then to STRIPE servers - not the hardest thing but definitely can be mundane.
With Next JS that issue was gone, doing all the transactions without 'an actual server'? Convenient to say the least.
Also, as to not bother making a firebase for this project, as I feel it would just be a filler, I allowed myself to simply store the data in the sessionStorage, not localStorage so to not clutter anyone's web browser.