Think of it as a back-to-school sales tax break, but for stoners. Colorado’s unusual tax law is forcing the state to suspend taxes on recreational marijuana for one day – Sept. 16 – during which a 10 percent sales tax and 15 percent excise tax won’t be collected. The sales-tax break would shave $20 off the price of a mid-grade ounce of pot in the Denver area, where ounces this summer sell for about $200 before tax. It’s unusual for a state that has many times rejected sales-tax holidays on things like school supplies, clothing or energy-efficient appliances. Officials say it could cost the state $3 million to $4 million. Retailers, meanwhile, are getting ready for an influx of shoppers.