Price that is calculated per transaction made and charged by the Payment Service Provider (PSP) to the merchant.
Transaction fees are mostly charged by payment service providers for their services (payment windows, interfaces, etc.). They can be charged as a discount or a fixed amount per transaction. Transaction fees are a part of the transaction costs that may be incurred for additional services such as fraud protection, verification, etc.
There is no single key here, as there are very different requirements that must be met. Due to the various components, the conditions should be negotiated individually in order to achieve the best possible sales target. Cheaper does not necessarily mean better.