Welcome to the community!
As far as I am aware, using additional data with that pre-paid data add-on would be charged 15¢ per MB (see image below).
~taken from here.
You should also note that the data add-ons are on a subscription basis (see Full details in link above). If you no longer wanted to use data, I believe you would need to cancel the add-on. You might consider contacting customer service to verify whether additional data add-ons are applicable within the same monthly cycle. If they are, you should note that they would likely be considered multiple subscriptions on your line. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
Hope this helps 😀
Hi there @kychat231 , if you used up all your data that you purchased, you would then have to purchase another add-on of data to use more. Once you use it up, no more data until you purchase another add-on. Hope that makes sense to you. You may consider moving to a post paid plan if you find yourself using more and more data, as that would be more cost efficient. A bring your own phone plan would give you a no contract account and pay monthly for data, text and voice. Then if you decide you don't want to have a monthly account, you can then cancel it. Also, any balance on your prepaid account, I believe can be transferred to your postpaid account. You would have to check with a Fido customer service rep to verify that.
Have a great day, OL