Hecto and Letterwell

I've recently signed up to both Hecto and Letterwell who help with paid advertisement for newsletters.

I've just hit 250 subscribers. It's a bit ballsy to already be expecting advertisers, but thought i'd sign up to see if anything comes of it.

It's impossible to put a value on subscribers so went for $20 per ad.
https://productivitypro.substack.com/

Has anyone had any experience with these sites?