To qualify for nonprofit postage rates, you must apply and be approved for nonprofit rate by Post Office. You may be designated as a qualified nonprofit by IRS but that will not allow you to mail at nonprofit discounted rates.

To qualify for the nonprofit rate, you need to:

1. Create a USPS account here.
2. Then complete form 3624. See this page for complete information.

Check here for more information on non-profit eligibility.