When I adopted Amber and Chilli, I remember asking the laedy in the RSCPA how they would like their donation paid (I was planning to donate £100-£120 for the both of them as they had been there 4 weeks and had vet care, spays, vacs and chips). The lady gave me a paddington hard stare and told me very sharply that it was a charge not a donation and it would be £60 for both of them. So when I went back I gave her the £60 and spent the rest on toys and a posh bed for the girls!
I guess it shows that all rescues are different. My opinion is that rescues should have a suggested donation, that way people can just donate that, more than that if they see fit or less/none if there are special circumstances, for instance if they were taking on a problem cat or one which would need a lot of vet care etc. I do agree with the holding fee idea, even if it was only £5, it may put people off lying on their forms and it would go a little way towards covering petrol and lost time if they should decide to lie anyway!