I've no doubt that's what's happened, but what they're doing (making regular donations, just as many other people and organisations do without feeling the need to make it part of their publicity machine) is not the same as what this email drop is not only implying, but actually saying ('paying membership fees', when there are no membership fees). It's sharp practice at best, and a cynical, misleading and downright disingenuous cunt's trick at worst.
The inference is that people paying the handsome advertising rates to be listed on their site who then register for Ugly Mugs are going to get something that others registered are not, the aim being to polish their image and get even more money out of us. They could have made a simple statement about being proud to support National Ugly Mugs (which is indeed run entirely on grants and donations) and put a banner link up - this is not just disgraceful, it's embarrassing.
Better still, they could have donated money because they want to help a good cause, and then just shut the fuck up about it like most of the people who have put their time, effort and money into the scheme over the last five years manage to.