When you become a patron to a creator who is charging patrons "per post", you have the option of setting a monthly max to your pledge. This is so you are never charged more than you're comfortable with if the creator posts a ton of paid posts in a given month.
Here's how to set a monthly max:
From the Dropdown menu in the top right select Your Pledges. From there click "edit" on the right hand side:
On the following page, select the checkbox next to "Optional: set a monthly max on your pledge". An option will appear asking you how many times per month you would like to contribute at max.
Don't forget to SAVE your changes!