The option to void is only available the same day that a transaction was submitted, prior to your plan's settlement time*. Voiding simply cancels a transaction and will save you any sort of processing fees or percentage rate based on the actual dollar amount that was run – so voiding a $1 transaction or a $10,000 transaction are exactly the same in terms of the cost to you.
A refund, however, can be run for the full amount or a partial amount, and for up to six months from the time that the transaction was originally charged. A refund will not create any additional processing fees, however, it will not negate any processing fees that were already charged on the initial transaction.
*If you have Next Day Funding set up on your account, voiding will only be available until 8:00 PM Eastern Time.