When to purchase JRPass

Hi,
We are traveling to Japan in late September, but would like to purchase the pass now. I understand that what will be sent to me via mail should not be more than 90 days before travel - so do I need to wait to purchase until 90 days before, or can I buy now and then receive 90 days before? If the latter, is there anything special I need to do when purchasing to indicate that?
Thank you