Whenever you use your Visa Credit card to purchase something whether online or in stores the merchant is charged a fee of 1.51% plus $0.10 per transaction.
The fee that the merchant is charged by Visa is most likely passed onto you the consumer through the purchase price of the product.
Since the merchant is charged a fee when you use credit or debit cards some merchants in their stores will give you a discount when you use cash instead of the debit or credit card for the transaction.