The Sale
When a merchant completes a sale and credit card payment is accepted the transaction is handled by multiple systems and networks that verify its authenticity and security through their platforms. Each entity (Franchise / Bank / Processor) charges a small fee in the transaction process like a toll for using its platforms also to ensure that the merchant receives the payment for each sale.
This route is different for each transaction therefore its cost may be slightly different for each sale. The variation depends on several factors such as the card type used to complete the payment, transaction amount, risk level, etc.