How about billing them openly and allowing them to opt out?
"Your bill includes donations to the following open source vendors, who made this work possible."
List the vendors and numbers.
If it is worded just right, if you make them understand that their product was made possible by open source volunteers, I think most people will not opt out, unless they really have to. (Some small businesses may have to, and that's okay.)
That's a very good point, opt in vs opt out of very interesting. Countries with opt out organ donation have much higher rates of said donations vs opt in countries like the US.
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