Need a digital converter box for your old TV? Don't wait too long to buy one.

These boxes will get harder to find as the switch to full-digital broadcasting takes place next year.

It's less than six months now until TV stations here shut off the old analog transmitters and go all digital.

KOMO News already broadcasts digitally. The analog signal will be gone on Feb. 17.

If you have an old-fashioned analog television that has rabbit ears or uses a roof antenna, those sets will stop working after the switch. You'll need to have cable, satellite or buy a digital-to-analog converter box.

Vince Munoz knew he needed converter boxes for his old analog TVs, so he got his government coupons and bought two units. But he didn't like the way the store rang up the sales.

The two boxes and two video cables he needed added up to $135.96. The store charged the 9-percent sales tax on the entire amount. The tax amount of $12.24 brought the new total to $148.20.

The two coupons were then deducted from this total, bringing the amount due to $68.20.

Munoz says if the value of the coupons had been deducted before the sales tax was calculated, he would have paid $61.

"And I talked to the manager and he said that's their order. They have to charge sales tax. Well, I said, 'I'm not very happy about this,"' Munoz said.

Munoz thought the store was wrong, so he wrote to KOMO News.

I contacted the state Department of Revenue. And it turns out, the store was right. He was supposed to pay sales tax on the full amount, and here's why.

The state considers the government coupon to be similar to a manufacturer's coupon because the retainer is reimbursed by the federal government for the full amount of the purchase. So tax is charged on the value of the coupon.

Here's something else to remember. Each household gets two coupon for converter boxes. If you've got two and they've expired, you're out of luck. The coupons for these converter boxes are only good for 90 days; it's use it or lose it.