If you ever get a response code 30 and “auth resp text” of “Invalid Cust Ref” it means you have an Invalid Customer Reference (ID). According to PayPal this field can be up to 16 characters and ALPHANUMERIC. If you put bad data in there such as an underscore _ you will probably get the response code 30 and “Invalid Cust Ref” error or “invalid value in message”. You may also get result code of 121 “Not enabled for feature” or a response message “Unknown error”.  The sale payment transaction will also be declined at the time of purchase.

Lesson of the story don’t put non-alphanumeric values into the Customer Code field AKA CUSTCODE AKA Customer Reference (ID).