I'd rather just donate to the charity in question. Why funnel it through a middle man when I could just donate it straight to the source?

If they're selling other merch and profits go to a charity, I evaluate it as a sale, not as a charity, and only buy if I want the item for that price.

People like to make the argument about the money going to admin costs instead of productive work, but I think that's silly because that admin costs will need to be paid by someone. If I trust a charity to allocate bulk funds properly, I should also trust them to allocate other funds properly as well. Money is fungible, so all that earmarking does is make their accounting work harder.