I don't see why this is a problem, so long as it's actually useful content for the question/answer. If you need to indicate a particular product in order to answer a question, it's useful to link to some source somewhere which has information on it, and most of those sources also happen to be vendors of merchandise. It would be rather difficult to avoid linking to any such source without losing some quality. In addition, advertising things on Stack Exchange sites (even your own products) is allowed provided it is actually useful and relevant and you don't do so excessively.
Of course, that doesn't mean all such questions are allowed here. We do allow questions on merchandise (with tag merchandise), but shopping recommendation questions (anything requiring a comparison of products or vendors) just don't work well on Stack Exchange in general. Such questions should be closed. How should we deal with questions asking for where to buy, watch, anime or manga media or such related merchandise? is the relevant meta post here on allowing (acceptable) merchandise questions. But if the question itself is fine, and a link to a vendor of some item is necessary or useful for an answer, there isn't any problem including one.