We do have a set donation, and we dont let cats go without it, direct
rehoming is a different matter, and we do have to hope that people will give the donation they say (I never ask for our set donation then, as I don't think it is fair).
I do like PB's suggestion though, I sent out an e-mail to people who had adopted from me at the start of the financial year telling them about fundraisers etc, and got a lot of donated food and some bric a brac, so maybe I should include one when I send out the newsletter, and send it to everyone, as I suppose if only one person responds with a donation, or an offer of help, it is better than we had before.