Hi again @Sean_Pritzkau and sorry for the surprise!! 
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Unfortunately in my country the Stripe service isn’t supported so the Buy Button is useless to me so far. Even, I can’t test it and give you a direct workaround.
I read days ago that items quantity option in shopping cart isn’t available yet so it is a problem currently
Maybe someone has a workaround/trick using the Buy Button but I don’t know so far.
This App created a similar Shopping Cart system but has some complex and to me, this method has some disadvantages:
https://deliveryjmc.glideapp.io
and here the owner describes how it was developed https://www.youtube.com/watch?v=OD2XVPE9FLU&feature=emb_logo
I use instead another different system to Shopping Cart without using the Buy Button, take a look at it to understand better(it’s in Spanish but some words are common in English like Orders (Ordenes): https://pizzeria.glideapp.io/ -
The way to make choices and other input fields as multi-user have some tricks and a litle more of knowlegment about Glide capabilities. Here we talk about the same problem you have Any thoughts on a Glide version of calculator? and this is my solution for these cases: https://multicalc.glideapp.io
I suppose I don’t have the final answers for your case (always a better/newer idea appears from someone) but in cases associate to multi-user problems, the way to go is what I wrote above.
Have great day, saludos!
Gavp