Well firstly I would like to speak on behalf of the employees in the small shops:

We are being watched, not you if we don't offer you assistance, we get into trouble...
So as annoying as we can be (I hated it myself before actually becoming one of THOSE employees) take a deep breath and consider the fact that our jobs are on the line, should we adhere to your unspoken wishes and ignore your presence completely.

Secondly, you do sound like you suffer from Social Anxiety, I believe therapy can help this, but as mentioned above, desensitizing yourself bit by bit will go a long way.

After each interaction, put in the effort to actually make a mental note of how that didn't kill you, even though it was more than uncomfortable.

These positive reinforcements will help future interactions as your brain will remember the previous mental note.

That's what helped me anyway... I had a brawl with what was for me, crippling social anxiety after I isolated myself for 14 months due to a severe depressive episode.

I was a social butterfly before that, but even so, it took me months, and months to not feel awkward, panicked and ready to run.

Practice makes perfect afterall.

Therapy seems like it could be very beneficial to you.