You don't carry a spare tube? I recommend that you carry the stuff you need to fix a flat when commuting...indeed for any ride. - spare tube, patch kit, pump, levers - that way you don't have to walk more than a couple of feet.
I can't tell you how much a shop is going to charge, I've never had one fix a flat. Figure 5 bucks or so for a tube, maybe 10 in labor.
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"Let us hope our weapons are never needed --but do not forget what the common people knew when they demanded the Bill of Rights: An armed citizenry is the first defense, the best defense, and the final defense against tyranny. If guns are outlawed, only the government will have guns. Only the police, the secret police, the military, the hired servants of our rulers. Only the government -- and a few outlaws. I intend to be among the outlaws" - Edward Abbey