How do I add published cards beyond my plan's cap?

On any paid solo plan you buy extra published cards for a flat monthly price: $4 a month per card on Starter, $8 a month on Premium and Max (yearly billing is simply the monthly price times 12, so $48 or $96 a year per card). The purchase is offered automatically when you try to publish past your cap, and you can add or remove cards on the Billing screen. The extra card joins your existing subscription and renews with it.