I think that another great way to support OS projects and the developers especially if you use them regularly is to actually take some time [ read money ] and contribute to it.
I know it's not always possible and that's where the idea of charging clients a small fee fits in well. I agree that this should be totally transparent, if the client is fine with the proposed price, 1% extra shouldn't influence his/her decision, if it does you probably don't want to work with such a company.
Hopefully makes some sense :)
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