There could be various reasons why you are being asked to pay. Let me explain each one of them.

1: Used Elite Asset (Image/Video) in your project: Elite assets are payable assets regardless of the plan. The premium plan gives you premium credit, using which you can only use premium assets for free. You can either choose to pay and download the video or swap the Elite Asset with a Premium or Free Asset to download the video for free.

Here is an article that details what Elite Assets are - What are Elite Assets

2: Exhausted Premium Asset Credits: When you use up all your premium credits and have used additional ones, you will be charged for them.

You can keep track of the asset usage by checking out this article - <Add Link Here>

3: Exhausted Premium Download Credits: When you use up all your download credits and want to download the video, you will be asked to pay $5 per export