I think that you should go for no.2 the second idea. Obviously not the first and the third is not practical..not everyone who looks on these pages seeking answers to their problems has even one credit card. I do NOT . And to discriminate between people who have credit cards and those who do not would fly in the face of the spirit of this site. I say this because it would not be long before you were forced to make it mandatory. Maybe it would be more successful if you were to make a call for ANY donations, rather than suggesting a certain small amount. This way you may get more.
I am sure that there would be companies prepared to advertise here, maybe rental car companies, airlines etc.But I think it would be ethically wrong to accept adverts from certain quarters on this particular site.