Since I'm assuming Amazon takes a percentage of the sales, it seems to me I would be better off donating the same amount [...]
Yes, as Amazon and Apple takes 30% of the price, and as VAT doesn't apply to donations, a much larger part of the money you'll spend will end up in the hands of Sweet Home 3D project if you donate. Thanks for your support! 
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Emmanuel Puybaret, Sweet Home 3D creator
