Any non-profit—no matter how small these days—will have at least one custom domain that they use for their online presence. I mean you don’t want to be running your org out of @gmail.com accounts, a Tumblr blog, and Facebook, right?
I inherited an account at register.com, and they were a complete nightmare. Super expensive, making you go through a live support agent for simple config changes, and constantly trying to upsell you. I’ve also had similar bad experiences with GoDaddy.
NameCheap has been great. I was using them for my personal domains (like this one!). I switched my org over to one, and also another community org that I'm helping with web management. They are inexpensive, relatively easy to use, and they've had reasonably good support.
Always check their coupons page before buying. You will get at least 10% off.
If you are a maximizer and would like a few others to consider, this is a good start. (Overall I’m a fan of LifeHacker, and they are one of my go-to resources when looking for a product category recommendation when I have no prior knowledge or experience.)
Also, Domize is my favorite tool for quickly seeing which name in the top 3 domains are still available.