Merchant account fee: Transactions have to be processed through a merchant account. Unless the terminal provider gives you a packaged deal with a merchant account included, you will likely pay a setup fee plus ongoing monthly or annual charges for it.
If you want to accept American Express, an additional merchant account usually has to be set up directly with Amex.
Monthly minimum service change: Many contracts require that you pay a minimum in transaction charges a month. If, for example, your monthly minimum is £25 and transaction fees only amount to £10, you will pay a total of £25 that includes the (less than £25 total) transaction fees you legitimately owed that month.
Reporting: You should as a minimum be able to view transaction charges and payments processed in an account online. However, some companies will charge a monthly fee for even the most basic functions like exporting sales to Excel or getting a paper copy of transactions.
PCI compliance: You may have to fill in PCI-DSS compliance paperwork in order to comply with card industry standards. This may be 100% your responsibility where you set this up yourself (can cost extra if support is required), or you may submit the documentation for free through your terminal provider.
If you have not set this up in the first few months, you may have to pay a PCI non-compliance fee every month you don’t have this in place (e.g. £9.99 per month).
Payouts: Some merchant accounts charge per payout batch, e.g. £1.50 every time transactions are paid into your bank account. If you require next-day payouts, this fee may be higher. In recent years, more card machine providers have scrapped the payout fee altogether, though.
Terminal replacement and servicing: Companies offering next-day terminal replacement usually charge for it. If faults fall under the warranty or the contract covers repairs, you may be able to avoid this cost altogether, but any urgent servicing is rarely free.