Purchase JR pass

I will be visiting Japan in April and planning to purchase a 7 days JR pass. How far in advance should I order the JR pass?

The website said that an exchange order will be issued after purchase. Will the Exchange order be sent via email or through postal delivery?

I am not planning to active the pass right away but I will need the airport transfer from Narita Airport to Tokyo station. Does JR company also sell airport transportation and how much does it cost?

Thank you and I hope to hear from you soon.

Regards