ModernAuthorize lets you take onetime payments very easily. Whether you are using the virtual terminal built in to Modernbill or using the one in the PSiGate.com management tools, ModernAuthorize will allow you to process those one time fees that you don't want to set up a whole order form to handle. Whether it is consulting fees, or a web design, or customer add-ons, you will need a solution that will let you be flexible.
Some third-party gateways do not have a virtual terminal. This means you will be required to set up an HTML page just to collect for that extra fee. This means extra and unneccessary support costs to you.