Mom keeps changing pickup times last minute and it's killing me

This is my third week dealing with this. I'll plan my whole day around picking up the kids at 5pm on Friday and she texts at 3:45 saying she needs me to get them at 7. I work retail so I can't just rearrange my shift like that. We have it written in the custody agreement that Friday is 5pm pickup but she acts like it doesn't matter. How do I handle this without involving lawyers again? We're on decent terms otherwise but this is getting ridiculous.