A pre-authorization charge allows a merchant to estimate the final cost of a product or service and request the customer's bank or card issuer to put a hold on that amount on their account.
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This hold is temporary and will generally clear on its own after seven days or less if the transaction is not finalized. The money hasn't been taken out of the customer's account however it is in their 'pending' to ensure that the money is there for the merchant to process if required. It also verifies that the card is valid and the amount is available to be used if the merchant finalizes the transaction.