I'm a member of Amnesty International and I was donating a sizable sum monthly, I also donated to other charities as/when I could (not lately as I am on medical leave for some months).
I would need to talk to my house-mate about this as she pays half the bill.
But generally, I like it. I would prefer opt-in option in the members area. You could set a minimum monthly donation (say $1) up to whatever a member feels like donating. Remember that life for many people is not very stable at present, and while they may be able to donate a small sum regularly, now and then they may be able to kick in more, or have to opt-out temporarily. The customer should always have the option. Given that you would have to administer the process, you would have to do your own cost-benefit-risk analysis also.
A reminder by mail or as part of the eMail newsletter would be a good idea. Most people have good intentions, but need a gentle reminder when life is hectic!
The sad fact right now is that many people are more worried about their grocery bill let alone donating to save endangered wildlife, or other good causes. Not because they don't care, but because they can't afford to.
I would also love to see something like a petition (for example) to the new Rudd Government to get them to match Exetel Customer donations dollar for dollar (or something like that). I think that might get more people interested, and help out the Wildlife Fund (or whatever charity Exetel & members want to support next). It would be a good way to build an "Exetel Community"
You could create a forum area for that kind of "Exetel Community Advocacy". Exetel have a large enough membership for that to be feasible.
Hope this helps.
BTW, I applaud your initiative in this.
The fact that your intention is to donate a part of what the customer pays for their service (which means that it's coming out of your pocket as it were, and will be a significant amount), and have an option for the customers to contribute something as well, is a good example of a Commercial/Consumer joint venture.
I am sure there are many members that would rather you donate the 50c (or whatever) to them (by way of a discount), but if 50c or even $1/mth for the Internet/phone bill is a big problem, they have some serious other problems to look at. I'm currently on a disability pension, which is peanuts compared to what I was making, and I don't mind the idea at all.
It's kinda nice to know I chose an ISP with a conscience and that gives a damn. I could have gone to Telstra, could anyone see them doing something like this?? Sure! After they jacked the prices up $5/Mth, they'd be happy to be good corporate citizens and donate 50c a Mth/customer! It's all the ACCC's fault you see! LMAO