The reasons below can cause the API error code to be returned: 

• The buyer's credit or debit card failed bank authorization
• The buyer didn't accept the Commercial Entity Agreement (CEA) before the deadline
• The final total of the (OrderTotal) significantly exceeds the original estimated amount you sent in the SetExpressCheckout API call

The buyer’s credit or debit card failed bank authorization
Notify the buyer that we can't process the payment and display other payment methods that the customer can use.

The buyer didn’t accept the Commercial Entity Agreement before the deadline
Verify the customer has accepted the Commercial Entity Agreement and submitted it to PayPal. If the buyer needs assistance with this, have them contact our Customer Support.

The final total of the order (OrderTotal) significantly exceeds the original estimated amount you sent in the SetExpressCheckout API call
If this is the case, the transaction may not have passed our security and anti-fraud measures. Verify that the API code passes the line item values correctly so that their total matches the OrderTotal value specified.

See the Developer Portal for a complete list of NVP/SOAP API error codes.