a1c47b6d_ation__NVP____PayPal_Developer__Field
[Description] (Required) Unique PayPal buyer account identification number as returned in the GetExpressCheckoutDetails response. PayPal also appends the payer ID as a GET parameter named PayerID to your RETURN URL when redirecting the buyer back to your website from paypal.com. Character length and limitations: 13 single-byte alphanumeric characters. [Field] 
PAYERID
DoExpressCheckoutPayment API Operation (NVP) - PayPal Developer
Field