MONROEVILLE, Pa. -- It's a story that may have consumers checking their receipts a little more closely.

A local woman is suing CVS, claiming the company overcharged her and her husband on sales tax. And she worries she may not be the only one overcharged.

The state is pretty clear about how sales tax works when you use a coupon.

Local consumer advocate Mary Bach is also familiar with the rules. When she said a local CVS refused to follow them, she decided to send a message.

For a company that promotes itself as offering extra care, a new lawsuit suggests that a CVS store in Monroeville hardly lived up to its promise.

Bach and her husband Lenny said they tried to resolve a mistake at the register.

"They simply would not fix the problem. They refused to take the sales tax off and suggested that we could go elsewhere," said Mary Bach.

The Bachs were buying a pair of digital TV converter boxes. CVS had put them on sale for $40 and the Bachs had two $40-off government coupons.

Just as the CVS flier said, the boxes should have been free. But they weren't. Their receipt clearly shows the Bachs were charged $5.60 in state sales tax. But in Pennsylvania, the law says the tax should be applied to the amount after the coupon is deducted, if the coupon and the item can be listed on the receipt, as they are in this case.

"I think I'd still go there, but yes, I will look at my sales tax now," said CVS customer Jim Henderson.

Worried that CVS registers statewide could have problems with coupons, the Bachs said they filed a lawsuit for $100, the minimum allowed, to send a message.

Mary Bach also successfully sued both Wal-Mart and Kmart over pricing and sales tax issues in the last two years.

"I think the customer needs to know that ultimately they do have some kind of recourse in these kinds of situations. And I think that my lawsuits in the past have proved that one person can make a difference," Mary Bach said.

CVS told Channel 4 that after the station's inquiry, it discovered its registers, nationwide, have a problem with the DTV coupons and that it's working to immediately correct it.

Anyone overcharged for sales tax can recover the money from the State Department of Revenue by calling its hot line at 717-787-1064.