Just a random thought: I’ve been using Joplin for quite some time now. Usually I buy some merchandise for software that I like and support. So if there is merch in the future, I would buy some for sure.
There was a thread some years ago, but I guess there was too little interest:
Side note: Obsidian seems to have a dedicated shop. Maybe its also possible to collaborate with some open source shop.
We have some nice enamel pins and stickers, but we just give them away at FOSDEM. I'm not sure there would be enough interest to justify a shop page and the logistic to create and deliver the items