This has been a long journey! When I first started the newsletter, and for more than a year after, there was no option of payment there as I haven't considered it at the time.
Initially I was using TinyLetter platform that didn't allow payments. This year, I think around February I've switched to Substack that has an ability to configure a premium paid layer of subscriptions which I did.
I announced that now there's a way to financially support DTO in one of the letters I've sent out - and hoped to get at least 3-4 paid subscribers out of the 1000 or so I had at the time. Nobody upgraded their subscription at that time :)
For now I'm making a point that a paid layer being similar to Patreon - just a way to support the newsletter and I want to put the money received into promotion/advertising of the newsletter, maybe doing book giveaways on Twitter and such. Later I hope to add more benefits for paid subscribers to provide more reasons to upgrade.