Payments are not handled by the cashier, it is a system where the customer can pay via a QR code printed on the product which only allows the user to view the details of the product and pay for it.
Yes the payment is in FIAT
Fake payments/ Theft is basically a situation where the customer picks the product and walks away without paying for it, I am thinking about how can I possibly stop this using cryptography.
Only product details and a predefined payment button after the QR is scanned are what I intend to be public.
The main goal of this product is that the cashier should be the 2nd option for the customer.
RFID tags are kinda useful in this situation, but not exactly. I don't want some employees to remove the tag from products after the payment is made so that it could leave the store. If the payment is made for the product, then the product/tag should be smart enough to detect whether or not it was a successful transaction.
I think you are overthinking it. Here is a solution:
Every item has a RFID. An app can scan the item and pay for it and register the payment in a database. When the customer walks out, the tag is scanned and the item's id is looked up in the database to determine if has been purchased. There is no need for a smart tag. The scanners do all the work. The tag doesn't have to be removed. In fact, you probably want to make it difficult to remove.
A cheaper alternative would be a QR code printed on the item, but that would be less convenient for the customer because everything would have to be manually scanned on the way out.
However, Amazon is already way ahead of you. They have stores where you just walk in, take what you want, and then leave. The store's cameras identify you and everything you take, and then charge your credit card.