Starting July 1, out-of-state online retailers selling in Massachusetts will be required to start collecting the state sales tax. The Baker administration estimates this new rule could bring in roughly $30 million for the next fiscal year.

But this extra cash isn't free. It comes with the added risks of a legal fight.

Seeking A 'Level Playing Field'

As a result of a 1992 Supreme Court decision states can only force internet retailers to collect sales tax if they have a physical presence in the state. And so with that logic, there's no debate when it comes to Amazon -- you already pay state taxes if you purchase, for example, paper towels on the mega e-commerce site. (Amazon acquired Massachusetts-based Kiva Systems in 2012 and operates warehouses in Massachusetts.)

But the same isn't true of other online retailers.

If you went to Overstock.com, for example, and picked out a new air conditioning unit, it would appear to be "tax-free" when you inserted your credit card details. Overstock doesn't charge sales tax in Massachusetts because it doesn't have a physical presence in the state.

You are still required to individually calculate and pay Massachusetts' 6.25 percent sales tax when you buy something online from a retailer that doesn't collect it, such as Overstock. You're supposed to do this when you file your annual taxes.

But, in reality, most people don't know that. And there's nobody actively enforcing the rules.

"It's really not the best way of collecting a sales tax," said Bill Rennie, vice president of the Retailers Association of Massachusetts. "For years, retailers here and across the country have argued for a level playing field."

Brick-and-mortar stores claim the current tax system hurts their businesses; people know they can buy the exact same item online and avoid paying sales tax.

"It's become much more important each year now with the growth of mobile commerce," Rennie said. "Folks can shop and comparison shop and purchase anywhere they want from their smartphones."

State officials saw the same trend and realized they were leaving potential tax revenue on the table.