Solo SaaS founder, customer wants a specific feature - how to prioritize / charge for it?

Hi there, I run an early-stage solo SaaS and recently had an existing customer ask about incorporating a new feature. In this case, it's something I think would be beneficial to other customers as well and could potentially help me win new business - to date I've taken the tack of, I'll take a few days to build the feature, ship it, and hope for the best.

However, I've done this in the past and have had the customer either go out of business and/or it turns out no one else really cares about the feature I built - and so it turns into lost time.

Is there a good guiding principle as it relates to prioritizing feature requests? Should I charge them separately for it as a one-off project? That also doesn't seem to sit fully well with me given that I'd also turn around and incorporate it as a selling point of my product eventually (either general availability or as an add-on).

Another related example, just today I had a prospective customer reach out and ask if we can integrate with this one third party API (which we did before previously for one customer for free, who then churned, so I haven't looked at the code in a long time). This isn't an especially popular platform relative to the other API integrations my product already has in place that I maintain. Do I simply tell the customer no given that there isn't critical mass for this platform, or do I in this case charge a setup fee in the event that no other future customers need this feature?

Thank you!