What damage can I be doing to my feet?

I am a florist and I stand all day at work (mainly 7-9 hours per day) but at busy periods I can be standing anything up to 15 hours per day. I'm also overweight and my feet and ankles get extremely sore. Mainly my heels ache. Is this causing permanent damage? How can I reduce damage ( except obviously loosing weight) and peak periods it's almost impossible to have breaks!