Read this to your customer
On today's date do you,    customer name    , agree to let us,   business name   , make a one-time electronic debit from your bank account for  amount of transaction  on or after today? If you have any questions about this debit, you can reach us at   phone number   .

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Recommmendation: Have repeat customers sign an authorization form. This saves you from reading the script each time and offers more legal protection since you have the customer's signature.