Problem
A customer applies a coupon code when checking out in a web store that runs EE 1.12.0.2 and is configured to use the Payflow Pro or Payflow Express payment methods. The following error displays:
PayPal gateway rejected the request. Field format error: 10431-Item amount is invalid
Solution
To resolve this issue, contact Magento Support and request the patch for support issue ID SUPEE-1474.
Then apply the patch as discussed in How to Apply and Revert Magento Patches.