Please don't get me wrong, its a good idea. I'm just trying to see the other side of the fence with this.
I wouldn't expose it to a client (in the way you state). Rightfully, why should/would they care about you wanting to give some money to a free library and ecosystem. A tool that you use to meet the end goal.
Personally, I would just let clients know as part of your proposition that you use OSS and donate to their upkeep and spin it as "By using OSS it allows quicker and safer delivery" or something to highlight the benefits. Whether you then up your base cost, or absorb it yourself (As you say, it could be 1%, no point in losing business over it)
Hopefully my rambling is somewhat clear! I get the enthusiasm, I really do.
Yes, I get your point and I think there's a lot of truth in it. Why should they pay a Euro more!? It's an experiment, though. Let's see how it will turn out!
I think one of the reasons I'm (trying to) do it transparently is because I can get an idea of how the company works and how the people working there think. If they're not willing to pay a symbolic euro for open source software they're using in the products they're making thousands if not millions of Euros with, I'm not sure if they're valuing other people's time and work enough that I will want to work for them.
I would almost say it makes the most sense to explain it all out like Rich says then either split it or match whichever you think would actually happen with the company, perhaps propose 3 options 1) you both donate 1% 2) you each split it each giving 0.5% or 3) you personally donate 1% on your own side alone but you could emphasize how great it would be for you to do one of the first 2 as you would be directly donating more for the devs to improve the software you are using or at least basing other work on.
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.
I get what you are saying, and I don't disagree with you. Myself, I think most clients don't understand what Open Source Software is, and honestly, they don't care. They come to us and ask us to do their project, we quote them a price and deliver the completed project. They just care that their project does what they need it to do. Now that isn't saying we should hide the fact we use open source. If a client asks, you can tell them, and then, as they clearly understand the concept, you can also say, "...And We support the developers whose tools we use." Otherwise, you probably don't tell them, "We develop your project on a Mac (Or PC)". If I have new business cards printed, they don't tell me 'We'll be doing your job on a Jamison XR-590r printer. The tools that are used probably aren't what matter.
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