Hi there Arboretum. Here's a rundown of how to set up the membership dues for your customers. What you want to do is use a non-inventory item. Just follow these steps to create one:
Select the Item List icon
Now select, New
Select Non-inventory item from the drop-down
Now, just fill out the remaining information.
Now whenever you make a sale for a membership, you can use the item you created. Unfortunately, currently there isn't a way to track when the next payment is due. Let me know if there's anything else I can do. Have a great day.