Is self-promotion in comments allowed or less frowned upon? The FAQ on Self-Promotion mentions answers, but doesn't say anything about comments. I've searched meta but it doesn't seem discussed, at least not in a separate question.
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Any such product or service promotion should be directly relevant to the OP's question. Any comment that doesn't clarify the question, or ask for clarification, can be deleted at any time without warning.
Overtly spammy comments can be deleted on general principle. In practice, spammers rarely bother to earn enough rep to post promotional material in comments anyway.