I'm a paid subscriber to a number of Substack writers, and I'd like to subscribe to more, but it can rapidly run into the hundreds of dollars per month - so I really have to pick and choose.
I can imagine bundles aren't really your model, and maybe aren't in your authors interest either. But I'd love to get like an Economics bundle and a New Zealand journalism bundle (shout-out to Bernard Hickey here, doing great work with his The Kaka Substack).
Or even a pay-per-article model - some of the free newsletters I'm on publish paywalled things I'm interested in, but I don't want to subscribe to the full deal. But I'd pre-load my account with $50 and pay a couple of dollars for a single article.
In a similar vein, LWN used to have a thing where people could pay to make the article free. There are sometimes paywalled articles I'd love to share around, but I don't want to deprive the author of revenue. If I could pay for a link that can be shared X number of times, or could chip-in to a "if enough people pay this article becomes free" fund, I would.