I saw that reoccurring charge possibility in Authorize.net and contacted them to see if it's possible to fix my error with a one-time charge. Here's the chat in case you were not aware. Maybe I'm the only one who will make this mistake but if others do you can let them know there's a pretty easy fix :-)

Welcome to Authorize.Net Live Chat Support! 
 17:49STEVENV:  18:01 I made an error, the people who have been charged $950 should have been charged $975 (they were advised of this pricing in the registration letter). I see it's possible to setup reoccurring charges using the card on file, is it possible for me to just charge the $25 ONE TIME using the ARB system so that it doesn't require the person to log back into our system to make the payment? This is Linda Karson, not Steven Volkers BTW
Dan:  18:02Yes it is possible Linda, but there might be an easier method to employ.
Dan:  18:02When did these 950 charges occur?
STEVENV:  18:04I'm all ears, trying to correct my error! I've been up since 2am when it was discovered. The charges have occurred over the last couople of weeks, I can get you the exact date if needed. It was only discovered today.
STEVENV:  18:05I found it, April 2 the letters were mailed
Dan:  18:05Well technically you can do this with any transactions that is not older than 90 days or younger than 24 hours.
Dan:  18:05Go to Transaction Search. Enter the date range, such as yesterday back to April 2nd.
Dan:  18:06then Hit search at the bottom.
STEVENV:  18:06Should I do this while we chat or just follow your instructions after we end the chat?
Dan:  18:07Then in the top right you will see View Rebillable Transactions of the list. Both. If you check the box at the bottom of the chat page, when the chat is over, we will send your transcript via email to the email address included when chatting in. This is to ensure you get the full transcript.
Dan:  18:07Then just to everyone that needs to pay and extra 25.00, add 25.00 under new payment amount and put a check mark in the check box to the left.
Dan:  18:08Also the transactions that are younger than 24 hours, can have this done on them tomorrow.
STEVENV:  18:09OMG that sounds super easy! Thank you, I've checked the box. That's just a one time charge then, right? I have fixed the system to now charge $975 but will check tomorrow. Thank you DAN! Linda
Dan:  18:09once you have checked the transactions that need to pay the extra 25, just hit Submit on that Rebillable Transactions and it will mass rebill everyone checked.
Dan:  18:09It is very handy and easier than the ARB subscriptions, which would be a one at a time thing.
STEVENV:  18:10I'll give it a try, thank you very much and to confirm this is just one $25 charge they will receive and I will notify them of the separate charge.
Dan:  18:11Yup. It will not be another charge of 950+25, just 25.
STEVENV:  18:11Have a great rest of the day, you've just made mine brighter :-) Linda signing off