I have been a member of Costco since they opened in the Seattle area back in the 80's and membership was limited to government employees, military and members of certain credit unions. I was a member based upon being a paramedic. I have never seen a sign and routinely carry concealed into Costco. If I boycotted every retailer with such a policy, I would probably be starving naked in a ditch.
Yup. For me, a business really has to be "in your face" about it for me to refrain from shopping there if it's a business that I otherwise really like. I would have to spend like half my free time researching which businesses are pro or anti gun (or anti whatever I'm pro or vice versa as well). I do lots of stuff with businesses and people that I politically disagree with. If they are anti-gun, unless they have some sign at the front with a beheaded gun owner, I'll politely disagree with their stance and just do my business there.
Plus, often it doesn't matter what store policy is if State law says otherwise. I occasionally stay in the Best Western within the Boomtown Casino in Reno when driving back from Idaho. They have a nice little "no firearms" sign at one of the entry doors (but only one). I routinely walk in packing concealed. In Nevada, "no firearms" signs have zero force of law. I could be asked to leave on general trespassing, and would have to, but not for legally carrying a firearm. Again a case of "concealed".
EDIT: I will say that I boycotted Hulu and cancelled my subscription after the Parkland shooting only because they folded like a cheap suit because of a couple of loudmouthed teenagers. Have an opinion for a reason. Don't be a lemming. Of course I was pretty "meh" on Hulu to begin with, so it wasn't much of a statement from me.