It seems like this should all be doable. I’m not sure how it is you are having customers fill shopping baskets – ie, I don’t know how you are getting your data into Airtable…
But, you need to incorporate the concept of “List Items” as a junction between a Product and an Order.
You have your Products table - each record (row) in that table represents a Product you offer, and presumably has a base Price attached to it.
You have your Orders table - each record (row) in that table represents an Order placed by a Customer (the Order “belongs to” a Customer, so you presumably have a Customers table as well) and has record of all the products and the total owed for the Order.
Your Line Items table will contain fields that link to an Order (a Line Item belongs to one Order), and to a Product (a Line Item references one Product), and probably has a field for Quantity, so that more than one of said Product can be accounted for in a single Line Item. Then you will have a Line Item Total field that multiplies the Price of the Product by the Quantity requested in the Line Item.
An Order may have any number of Line Items, and the Orders table will have a field that sums all the Line Item Totals for all Line Items linked to it to get the Order Total. You can then have a field where the Coupon amount is entered, and another formula field that can compare the Coupon amount to the Order Total, returning some warning message when the Coupon is exceeded by the Order Total.