Pickup is always 15 minutes late and I'm over it

Every. Single. Friday. My ex is late picking up the kids. Not by like 5 minutes, we're talking 15-20 minutes regularly. Then she acts like I'm the one being difficult when I mention it.

I have plans sometimes, man. Or I just want to have my evening. Am I being petty if I charge her for my time or just enforce the agreement strictly? This has been going on for 2 years so it's not a one-off situation.