My 2 cents here. As a freelancer, I assume that you are a contractor and go through a quoting and billing process? I'd suggest a revision of terms on your quote that says that when using OSS, you contribute a reasonable amount to the project for use and that in cases where this use applies specially to a project this will be billed as an expense. You should do something like this with any paid special use libraries that you use as well. If 20% of your customers need a 200 euro/year tool you should only pass that cost to that portion of your customer base (either through rates, direct billing, etc).
Philosophically, I agree with your approach if you use roughly the same packages on each project as part of your toolkit and they create value and savings for you clients. In that case, your value on a per hour basis is being enhanced by the OSS and it makes sense to pass that on as such. Where it makes less sense is when and if you have significant variability in what packages are used, the extent of the package use, and the value proposition to the project.
I haven't done freelance in many years, but honest transparency makes sense. The customer has a interest in the continued support and availability of the tool chain and we should justify it as such.
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