Maybe ask yourself why you wouldn't give 1% of your revenues to open source projects. After all, you decide to use them and you make money off them. Like Rich said, the client doesn't really care and might go with another developer that doesn't charge that extra %.
It doesn't prevent you from being transparent. The pitch is even better: "By choosing me, you also contribute to open source projects because I will donate 1% of the revenues generated by this piece of work".
It doesn't matter if your client is making thousands, millions. If that's the case, then you should sell your services for more, which makes your own donation even easier.
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