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0711mariechen,
welcome to our community.
It’s nice of you to want to exchange knitted items for the cost of materials plus postage and give your working time for free, but I think if you want to do this more often / regularly, you may need to register a business. After all, these are not items made for friends, but for people on the internet whom you don’t necessarily know. To be on the safe side, it might be a good idea to talk to a tax advisor about this. The tax advisor can tell you how to handle this sensibly.
I wish you every success, and please let us know what you decide and how things go for you.