X is the only platform that charges Postatron for every post, so it is the only one with a monthly cap. Once the cap is reached, new posts to X wait until the count starts again on the 1st of the month, while posts to every other platform carry on, and posts you had already scheduled still go out.
Choose a plan on the Plans page. Before anything changes, Postatron shows what you will be charged today and on your next invoice, worked out by Stripe. Nothing is charged at the click: the difference goes on your next invoice, as a charge for an upgrade or a credit for a downgrade. The new plan's limits apply straight away.
Cancel from Settings, Billing, Manage billing. Your plan carries on until the end of the period you paid for, and you can undo the cancellation until then.