Narita Express on delayed basis?

Hi, Today when I wanted to reserve and purchase N'EX tickets, Ofuna to Narita, for several days from now (this was at a small-town eki), the JR person did the reservation but after a long careful study of my rail pass told me there would be no charge for the N'EX tickets. I am almost certain he understood that my rail pass is expiring tomorrow, so his decision not to charge for the tickets seems too good to be true. But it does seem conceivable that as a side-deal for buying a rail pass you get to finish your travel with a free N'EX to Narita. Can you confirm my suspicion that despite what the JR agent told me, I will have to pay for the N'EX? Thanks in advance! - Paul