Payment processing solutions (such as Plug 'N Play® or Authorize.net®) are simply the "gateway" or middleman connecting you (the merchant) to the bank where your customers credit cards actually get processed. By providing you a user friendly transaction-processing system (ie: virtual terminal) to utilize and process orders thru, they charge you a few fees.
Gateway / Virtual Terminal Monthly Fee – This monthly fee is for use of the terminal and generally ranges from $20 - $40 per month.
Gateway / Virtual Terminal Transaction Fee – This fee is a per transaction fee and generally ranges from $0.20 - $0.35 cents per sales transaction. Yes, this is a 2nd transaction fee (in addition to the transaction fee you pay to the bank). This one goes to the gateway provider.