Relax. Same thing happened to me. I must have called Heather half a dozen times too! Long story short, it never showed up and they had to write/sign a new one and resend it. It finally showed up and I was happy.
You can call and ask Heather when the letter was sent out. Then do your own calculations to see when you should get it. Give it a few days longer than that, then call her back and tell her it didn't come (unless it does
😀)
I'm sure you're fine. It happens.
Personally, I suspect my mail carrier of trying to sabotage my attempts to enter med school. I often wonder how many other interview/rejection/acceptance letters failed to materialize due to her jealousy of my future career.