The logo is great but it has a Creative Commons Non Commercial - Share Alike attribution so I'm not sure if what we're using it for would be considered commercial. We could always contact them to confirm though and I don't see it being a big problem at all.
Yes, good investigating! On this page, www.accessibleicon.org/take-action.html, I found a statement that would seem to OK for stickers. "We have included a free PDF of the Accessible Icon in our shop. Feel free to use this PDF to print off stickers, create parking lot symbols, or maybe even order a batch of temporary tattoos! Use your creativity to create community and promote accessibility." But if using for t-shirts to raise funds, would be advisable to ask first and get written permission.