Read the bottom of the donate page where it says You can get 2 keys per each 3€ you donate to unlock advanced features such as automatic symbols downloading, remove donation notification or remove desktop watermark. If you look closely, that's not a dollar sign - it's the Euro sign so you have to do the currency conversion... €3.00 = $4.18 (currently). Basically you have to donate at least $4.18 to get two keys - I don't think you can donate half to just get one.