Windows applet programmer w/ Zoho Books experience

Scenario: Products we sell are in high demand and short supply. Some customer store owners want as much product as they can get, and to ensure that they express their intent to buy. They do so by placing what is called a pre-order, but they do not specify a particular item, only that their purchase would be any collection of items in our product line. They do this because we are never sure which products will come available sooner than others. So this applet utility would be to track which customers are in pre-order mode, so when product comes available we could select one of those customers in the list, select product items and quantities to be invoiced, and populate multiple JSON invoice files to send to Zoho Books so it can create the invoice within its system. CUSTOMER ONE might have 12 invoices generated, one for each product item selected if we had selected 12 product items. You might ask why each product needs a separate invoice. We don’t have an answer for that except that it’s the way they do things here. Something about regulations or whatever. We might also want to toggle into a mode where we can select several customers, and several products, and have the applet generate one invoice for each customer for each of the products. So the customers and products pick lists would be populated through http calls to Zoho Books, but the tracking of customers preorder status would mean keeping a local list (customer ID#) of those customers who are preorder status. We don’t need to store any local product lists because those only need to go directly to the JSON which would get sent to Zoho Books as invoices are generated, since we won’t know which products until the last minute.