Coparenting Challenges

I'm trying my best to make this divorce work for the sake of my children.  They know that if they need anything that I can provide (besides my physical presence) then it's theirs.  With that said, I've made a few "pop up" visits to the house to drop off miscellaneous items the girls needed.  Sometimes I would knock and put it in someone's hands.  Other times I would leave the item and simply send a text message.  Apparently my ex-wife disapproves.  She sent me an email a few days ago that I only noticed by chance tonight.  She basically said to stop showing up at the house unannounced.  She said I know when my visitation times are and if I need to see the girls outside of those times, I have to inform her.  Cool. No problem.  I can handle that.  We've been divorced 3 months and just when I thought things were getting better, I realize they are getting worse.