This is a public question forum, you will need to contact them directly for questions like this.
It is highly possible that they might not charge you because the cats are not yours. I strongly suggest that you go directly to the shelter and speak to someone at the front counter, call them or go to their website for more information. Or you can also leave them in a crate outside of the shelter's door when they are about to open for the day. In this way, you don't risk being charged for leaving them there.
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