Our small town post office had a new ramp and curbs put in but they never painted for easy visibility for over a month - so i finally wrote a short note about accessibility & visibility {tripping hazard} and put it in the drop off box - 2 weeks later it was painted.

I was really surprised that a gov agency was lax on something like that.

Some times a note or call to the local store manager might be all that is needed.

At the very least a sign saying "no public restrooms" or such would have been nice.

I have a nervous tummy/bladder at times so I know how you felt.
I tend to shop at stores with handy restrooms myself.