A customer enters a store and asks if he can pay with cryptocurrencies, the merchant says that ok , and at that moment the customer clarifies that he bought with the currency X, the merchant should check the price of the product in his original currency, make the conversion to dollar or Euro if it is not already in that currency originally, then validate its conversion to currency X, to give the price to the customer, if he accepts the price, the merchant must open a portfolio in that currency to receive the payment, from there everything that corresponds to the merchant is triggered by that collection, such as taxes, invoicing, inventory control and etc.
As we can see that is unfeasible, assuming that the transfer is in real time. which does not always work like that.