Stack Exchange questions are the highest-intent technical content on the web: someone described their exact problem, and thousands of people with the same problem found it afterwards. It is also the least forgiving surface here — the community has spent fifteen years removing promotional answers, and it is extremely good at it.
The rule is short: the answer has to work without your product. If the question can only be solved by buying something, say so and disclose the affiliation; if it can be solved another way, answer that way and mention your tool as an aside or not at all.