> Nothing wrong with the advice, but most subreddits have a no self-promotion policy

So, instead of "Go to my site to buy X!" you just talk about X in the third person. You mention your product/service X alongside the obvious brand names, you always have a story ready about how X can do what OP wants, etc.

The self-promotion rules just force you to be dishonest at all times.

Works well because people have a knee-jerk hate for "check out this X I made" but they are completely blind to "check out this X I found." You can see this a lot in r/gaming when solo devs share their work.