The {{AmountPaid}} value is just one of many 'Response Tags' that are available to be added to the Confirmation Email on forms. To see a full list for your particular form, got to the 'Confirmation Messages' tab and edit the 'On Screen Message'...

Next, click the 'Response Tags' button below the 'On Screen Message' text area...

Then you can copy any of the available response tags from the scrollable lists displayed. NOTE: The 'Primary Tags' list contains the standard values common to all forms. The 'Answer Tags' list contains the answers to specific questions on an individual form.
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