Portion of the transaction amount which is sales tax.
The sales tax charged must be included in the TransactionAmount of the original transaction.
This amount is to be presented with an implied decimal point. For example, US $10.00 must be represented as 1000, and $0.10 is likewise simply 10. The maximum length of this field is 6 digits, thus the maximum AmexSalesTax is "999999", yielding a US dollar amount of $9,999.99. This field may not contain a negative number.