Marketing isn't a bad thing on Hacker News. There are behaviors associated with marketing that HN deals with efficiently. These include getting attention by an annoying number of posts. I think you're probably far from that. A few posts a few days apart is unlikely to be considered spam. If you're concerned about overstepping HN norms, maybe contact the moderators using the |contact| link.

Show HN is explicitly designed for people like you to market their projects. It seems like you value HN, but are maybe erring too much on the side of caution.

For what it's worth, your conclusions about what a large market means may be causing you to avoid finding customers one a time. Big established markets mean that most potential customers already have something that lessens the problem. They're not out actively looking. The recurrent cost -- a $5 credit -- is often not the major cost. Time spent figuring out if your product meets their needs is more important...I mean, five bucks is nice, and consumers may switch for it. A well run business spends more money in salary when an employee looks at the landing page. Deploying a new app at scale means changing a business process. That's where a new app fits into the ecosystem.