Actually, it's probably not sales tax.  You may be seeing the "Offer Cash Discount" option in effect for credit card purchases.  Choosing the Offer Cash Discount option allows you to offer a lower cash price.  By law in most states, the credit card price is what must be advertised.  This app will adjust the price based on Cash, Credit or Invoice payment if the cash discount option is turned on.

To enable or disable the "Offer Cash Discount" option. Tap on App Settings from your menu, scroll down to "Offer Cash Discount" and check or uncheck the box based on your preference.

Checked = Credit card processing fees are added to your client's total

Unchecked = Credit card processing fees are deducted from total when Propay sweeps your funds into your bank account.