I go into Starbucks...yes, I shamefully admit I sometimes pay rediculous prices for a Frappacino..
and Starbucks is known to be liberal and given support to gun control. Yet they don't have any "NO FIREARMS ALLOWED" signs and I have walked into their stores a couple of times with open carry and never been bothered by anyone. I have gone in with CCW as well.
So if liberal Starbucks can tolerate my carrying a gun in their store and appreciate my business more than their personal feelings or agenda, then Costco should learn from them.