Marketpress special instructions did not go through

I am using Marketpress and have a 'Special Instructions' field on the Shipping Page. I just moved to 'live' and placed an order through PayPal Express Checkout. Everything worked perfectly except that the PayPal receipt clearly reads: "Instructions to merchant: You haven't entered any instructions."

When I manage orders through my dashboard, my special instructions field clearly did work correctly. Any suggestions on why PayPal says otherwise?