Agoraphobia can be part of the PTSD. Unfortunately, strangers really are not going to understand or comprehend the "place" you are in and you can't expect them to.
It's wonderful you have a service dog to help you. Do you like music? If you do --- take a ipod with you and play your favorite music. This allows you to focus more on your music than your surroundings which could help you to relax. This is not a miracle and can take several tries before you see any benefit.
I had someone with me when I first started to try and get out but people have their own lives and I could not expect someone to change their lives to suit mine.
My therapist suggested going into stores or anywhere where there is alot of people. Before I left my car I would put in my head phones and find the music that I wanted. It was so hard and scary, I hated going anywhere but I knew I needed to work through this.
I probably spent about a year where I never left my house without my music, it went everywhere I went. Little by little I did do better getting out, although even after 10 years I still sometimes struggle to leave the house...but I do.
I hope in time you can find peace in your life