We also have charities that ask for donations on television and online. Although many have suggested giving levels, I don't think we have any that only accept specific donation levels. I think all of ours are happy to accept any amount you care to donate.
I donate to (1) our Georgia Public Broadcasting (GPB; affiliate of Public Broadcasting Service, PBS); (2) Northeast Georgia Food Bank; (3) Humane Society (animal welfare); and (4) Toys for Tots. I also occasionally donate to Habitat for Humanity (most recently, I donated a working clothes dryer that was only a couple years old). What they all have in common is that they will accept whatever monetary donation I feel is appropriate, and although they ask for donations each year, don't guilt me into donating what I can't afford (my annual income is below our national average, I still have a son in college, so I spend carefully). Charities that use guilt as a tactic don't receive anything from me. Same story for charities that are unrelenting in their fundraising (I once made the mistake of donating to Doctors Without Borders; never again, even though they probably do good work).
The charity that has received the most for me is GPB / PBS. The BBC could learn something from them. 85% of their funding comes from completely voluntary donations, about 15% is federally funded. Who receives their broadcasts? Everybody with a television or a radio, for free, even if you can't pay. I have been a member since 1987, and most of those years have been as a sustaining member. Charity Navigator gives them a rating of 97% (four stars) based on the charity's financial health (including financial efficiency and trustworthiness) and its commitment to governance practices and policies.