I thought that you only get charged coin when you actually step through the warp portal that you make?

I made a mistake when choosing the portal location and closed it, but I still got charged coin for it.

Yea you get charged once it starts glowing orange I’m pretty sure

Are you sure it was a portal? As portals use Oort Shards, whereas it’s the Warps that uses Coin, so presumably you were talking about Warps.

If you’re opening a Warp from the sanctum, then you will not be deducted any Coin until you walk through it. However, if you’re opening a Warp on a world, then you will be deducted Coin once you select the Open Warp option.