Yongki is great with this kind of apps! 
I believe it’s this one! : Let's build Online Ordering App (clone-able)
I think you have setup-ed this correctly! And in your pay button action you don’t need to update order items property because you are updating that property with the Join Now button. I mean when you create a new order item with the current order then that order item is adding to that order automatically.
Just updated the app! : https://powerofknowledge.adalo.com/paid-content-platform-clone
And thought to add this great post by Flawless too! : Cart issues - i can't remove object from my cart - #3 by Flawless
Thank you