Transaction fees are fees that a processor charges the merchant associated with each credit card transaction. Some examples of transaction fees include authorization fees, return fees, AVS fees and gateway fees. More than one transaction fee may be applied to an individual credit card transaction.
A per transaction fee is an expense a business must pay each time it processes a payment for a customer transaction. Per transaction fees vary across service providers, typically costing merchants from 0.5% to 5.0% of the transaction. (via Investopedia)