Does this Substack-alternative already exist?

A newsletter platform that matches advertisers with writers?

Not sure if this service already exists. Would be great if it did.

Idea came via Peter Cooper: https://www.indiehackers.com/post/managing-10-newsletters-with-peter-cooper-of-cooper-press-6f4083efd1

Peter says podnews.net has "nailed" the classified advertising process with its self-service classified ads: https://podnews.net/article/advertising

So what about a Substack-type platform that operates a self-service ads process, where advertisers can find suitable newsletters (and therefore audiences) in which to place an ad.

The advertiser could select multiple newsletters, write different ads and bid different prices for an ad slot on each.

The newsletter writer meanwhile prepares the newsletter and just before sending, can choose from any submitted bids for that newsletter to include an ad. If an ad is included, payment is made. The platform of course takes a cut of the ad payment.

Ideally this platform would cater for small-time newsletter writers and advertisers, where ad bids could be just a few £/$.

Has somebody already built such a platform?