Good question! And thanks for seeking feedback here as it helps other users too.
This can be a tricky issue because it's often subjective (and therefore users are influenced by their personal experience, feelings, tastes, or opinions).
In this case, the question itself could have been closed on a number of grounds but obviously the community didn't think it quite crossed the line, so it still stands. To my mind you've answered the question and the community by and large has agreed by the number of votes your answer has received.
So, in a nutshell, you haven't broken any promotional rules.
However, to help reduce the risk of users judging an answer as being promotional, users can take some steps. Using your answer as an example, users can:
- Be less emotive. So, instead of "You can also purchase at great prices from them." it may be better to say something like "Generally they offer competitive prices."
- Use a clear disclaimer. Have a clear disclaimer that declares your interest, if any, when referring to a product, service, website etc. Something like "Note: This is not promotional just trying to provide a site." is not as clear as something like "Note: I have no affiliation with..." or "Note: I happen to work at..."
- Offer multiple recommendations. Where possible, provide more than one choice so that it's less likely to be judged as purely promoting a particular product, service or website. If you've only used one personally, you can specify that so it's clear you haven't used the others.
As I said this is often a subjective topic. Some things are very clearly promotional (or even SPAM), but others can be caught up in that grey area of uncertainty.