I live on a farm and my 18 month old GSD is free as a bird. Lately, she has been tracking me when I leave. How Should I continue using the collar?

I have a 18 month GSD. I live on a farm and Lili is free as a bird. She is very devoted to me and quite obedient, very loving and likes everyone. However, due to illness in the family, I have not been home as much as usual. She tracked me into town (about seven miles) one day so I lock up her up. Last week, I had to stay over night and could not lock her up--so she had tracked me again and visited a neighboring farm. They identified her from her collar and brought her home so she was here when I got home the following day. She has been running after the car when I go to the mail box so yesterday I put the electronic collar on her (never have used it on her before) "zapped" her when she began following the car. Today, she remained in the yard (thank goodness) and didn't follow the car. Should I continue using the collar or do you have a better solution? She is a "hard head"--not sensitive at all but is a great dog--I am a gardener with many shrubs, roses and perennials with dozens of deer waiting to destroy all --she is great at keeping them away besides being a great loving companion. Any advice will be appreciated.

Corine