It's probably a remaining balance owed from a past Redbox rental. The complete charge from your rental is on your email receipt and says Total Charge. If you still have a remaining balance from a rental because we couldn't collect payment you'll see it on the line of your receipt that says Balance Due. (If you didn't get a receipt, it's because you didn't enter your email address at checkout.)
Balance payment is an automated process. Redbox collects the funds owed when they're available in your account. The collection date can't be changed.