Most internet merchant account processors will charge a $25 Monthly Minimum - which means if you’re not processing around $1200 a month in Visa/MasterCard transactions, you can be charged an additional monthly fee up to $25 for your credit card processing.
Many internet merchant account providers require you to sign a 2-3 year contract - and then if you cancel early for any reason, most will change you a $200-$500 early termination fee.
A lot of other credit card processing companies (especially those offering you a “free” terminal) will charge you a yearly fee that ranges from $30-$200. They’ll either call it an annual fee, membership fee, or a risk assessment fee.