You need to get on a Volume License agreement, specifically one designed for 'Not For Profit' organisations.
The best way to do this is to use an accredited MS reseller or a company whose distributor is accredited. They will need to setup the agreement for you and you'll need to provide some evidence of the entitlement.
What it will all boil down to is you'll get pricing very similar to the Educational license prices.
If the Charity is high profile enough, they should also hassle their Microsoft account manager for a grant of licenses. MS are reasonably supportive in this area, in my experience.