Using grants to make donations to institutions for using their resources/products?
It is ethical to pay the price you are asked to pay; I'm not sure if it's ethical to get a funding agency to pay more than the advertised price, and your example could be understood as suggesting precisely that, even if the cause is good.  I've never heard of an agency allocating funds for donations (imagine the potential for corruption!)

I personally feel there is nothing unethical about using a free product.  Academics have many occasions to give back to the community without formal compensation: refereeing papers, editing a journal, reading theses, sitting on grant selection committee, organizing or participating in outreach activities etc. It's all part of the broader duties that come with job, and for those who care to keep score I found in the longer term that service roughly evens out: you contribute for free roughly the same you get back for free.