Sorry to hear that Mike!
I hope they realise what a mistake they've made!! Obviously you didn't intentionally walk in in oily boots to leave marks on their floor. AND you apologised!
It's sad but true that some people judge you by what you are wearing.
I've found that out after working with horses, and in wellies, jogging bottoms and smelling of horse s**t you definitely get treated differently. It shouldn't be necessary to go home, have a shower and get changed, just so you can go into a shop and get treated like a normal person!!!
Also found being a female going to a dealer to get parts gives you funny looks and questions. I much prefer going to a local scrapyard where the guys couldn't be more helpful. They know my car now, so when I stop outside they're already there asking what I'm looking for.