The Ad Hoc plans are one-time payments and cover the batch of emails that you upload at a specific time. They are not recurring monthly plans.
You can upload multiple lists in one batch. To do this, upload your lists one after the other but do not proceed to validation until you have uploaded all lists that you have. Then click the button to proceed to validation and it will add up the emails from all the uploaded lists as one batch and you can make a single payment for all the lists. The lists will be verified separately and not merged together.