Question : Why does it appear that my customers credit cards are getting double billed ???

Ans: The credit card is processed in two stepsStep 1: is the authorization of the credit card that sets aside that amount of money until the merchant takes the second steps.

Step 2: is closing or settling the batch ( that happens over night )

Once the batch is settled the credit card issuing bank sends the money to the merchants bank so no money exchanges hands until a batch is settled which is why you can only void transactions in the same day they are initially submitted.

By voiding them you are removing them from the batch so the customers credit card bank never gets the go ahead to send the money,however the customers credit bank will hold open the authorization for a few days to give the merchant time to settle their batch.

In some cases a merchant like a restaurant will put through an authorization for more than the amount of the bill to cover the tip which is added after the customers credit card has been authorized or in some cases by merchants providing a service whereby they do not know the total of the transaction prior to providing a service so they authorize a higher amount then after they do the service a day or two later they complete the transaction and settle it.

Q How does the bank message a customer saying that you've been charged even if he isn't ?

Ans: Most on-line banking will display authorizations and some will even show the amount being deducted or added to your balance because they are assuming that the settlement will come through it's done to protect the integrity of the initial authorization and this protects our merchants as well.

Example:-

lets say that Merchant-1 sells something via a credit card for $100 and he gets back an authorization on the transaction. Let's also assume that the customer only has $101 available prior to the authorization and $1.00 after it. Now let's say the card holder goes to buy something else in an hour for $50 at that point his credit card would be declined because the bank has "deducted or allocated the money" to the Merchant-1.

Now let's say that the bank didn't do it that way and that they approved both transactions and Merchant-2 settled their batch before Merchant-1 don't you think merchant 1 would be a little upset if there was no money left for him. Hence the authorization response.

Question: Why does double billing happen in the first place ?

Ans:The only reason for this is a coding error.