I think they can afford free shipping via a couple of ways. First, there's book rate with the USPS that's used to ship books and reading materials at a discount

I think they can afford free shipping via a couple of ways. First, there's book rate with the USPS that's used to ship books and reading materials at a discount. Second, since they're a nonprofit, albeit a B-Corp, they might qualify for an additional discount since they're a volume shipper.