I applaud the step to ask for donations prior to the download. It totally makes sense. I also like the humble-bundle-style sliders. I'd really like a slider though, that allows peope to give a humble tip to Debian who - after all - make Ubuntu possible.
closed as off topic by RolandiXor♦, Mik, Eliah Kagan, andrewsomething, Stephen Myall Oct 11 '12 at 20:41
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If you want to donate to Debian, it's better to give directly to SPI, a non-profit which handles donations for them. If you donate that way, it's tax-deductible for you, and I don't believe that SPI pays taxes on it either. Contributions to Canonical are not tax deductible for you, and Canonical will have to pay taxes on them as well.
You should file a bug report on this asking for it:
In the meantime you can directly donate to Debian via SPI: