Let's get hypothetical here. You bought 150 cartons of cigarettes via the Internet over the last two years, and last week the state Department of Revenue sent you a bill for all of the back taxes and fines you owe. Lousy break.

Meanwhile, your pal bought just as many cigarettes as you did -- but because he bought them from a different Web site, one that doesn't abide by the federal law requiring cigarette vendors to share buyer information with the 50 states, Pennsylvania didn't find out about his purchases, and thus didn't tell him to pay up.

And the guy who buys 50 cartons didn't get a tax notice, either, because he never reached the state's rather arbitrary 100-carton threshold for notification -- a level it set because it assumes people buying that many cigarettes probably are reselling them, not smoking them.

Fair? Probably not.

Constitutional?

That's an excellent question.

When it comes to matters of taxation, the courts in Pennsylvania typically are asked to apply three tests -- does the tax-collecting statute meet the U.S. Constitution's equal protection clause? And does it satisfy the state constitution's exemption and uniformity clauses? The U.S. equal protection clause says, essentially, that a citizen should be "treated in the same manner as others in similar conditions and circumstances," according to Cornell Law School.

The state's uniformity clause says "all taxes shall be uniform upon the same class of subject [and] shall be levied and collected under general laws." The exemption clause lays out which entities -- churches, charities and so on -- are exempt from taxes, but that wouldn't apply in this case since it generally refers to property taxes, not excise and sales taxes.

So that leaves us with uniformity and equal protection. We'll deal with uniformity first.

The tax rate certainly is uniform -- everybody who buys the cigarettes is indeed supposed to pay sales tax and the $1.35-a-pack excise tax. But the actual practice of tax collection doesn't seem to be uniform at all.

When the state targets certain buyers over others in collecting taxes, isn't that an uneven application of the law?

Or does it make sense, both practically and legally, for the state to use its enforcement resources to track down the scofflaws who owe the most money?

"The uniformity clause applies to legislation, rather than enforcement," said Duquesne University law professor Bruce Ledewitz, an expert in constitutional law. "But there's no normal situation in which there is any distinction between legislation and enforcement."

So this situation isn't normal. The Department of Revenue, thanks to a federal law that is specific to tobacco products, has been receiving buyer information from certain online and mail-order cigarette vendors. It then uses that buyer information, which includes the number of cartons purchased, to track down the buyer and notify him of his tax debt. The buyer owes $13.50 per carton in tobacco taxes, plus the 6 percent or 7 percent sales tax, plus late fees and daily interest.

More than 4,300 Pennsylvania buyers were targeted last week. The Revenue Department says these buyers alone owe $10 million in uncollected cigarette taxes since January 2005.

The state says it is targeting the people who have purchased at least 100 cartons because it figured anybody who bought that many cigarettes might be reselling them for profit.

But plenty of people hit with the unexpected bills have said that was not the case. A couple from Washington County, who bought cigarettes for their own use, will owe about $3,000 in taxes and penalties. An 82-year-old Pittsburgh-area man bought more than 400 cartons over the years; his excise taxes alone will come to $5,000. Sales taxes will be another $500 or more. Penalties could be $3,000.

Yet another woman told the Post-Gazette she owed $3,854. The state told her she'd have to pay 10 percent upfront, then about $150 a month over two years, to work off her debt. When she told the state she couldn't make such a large payment, she said she was informed that the state could place a lien on her home.

"That's a violation of the spirit of the uniformity clause," Mr. Ledewitz said. The effect of the 100-carton threshold is that "the tax is zero up to 100 cartons .... The tax is obviously uniform as written, but it isn't being enforced in a uniform fashion."

Getting a court to agree with that, though, is another matter.

"I think that one could bring a uniformity challenge," said Robert Strauss, an economics and public policy professor at Carnegie Mellon University. "But I don't think the argument is a slam dunk."

The state's first line of defense would be that it's targeting higher volume buyers for reasons of administrative convenience.

The next defense might be that there are other loopholes already in use -- the state, with congressional blessing, doesn't collect sales tax on purchases made with food stamps, for example.

"You've got some people paying taxes, and other people not, on roughly the same purchase," Mr. Strauss said. In other words, there's no uniformity, yet it's constitutionally OK, since congressional law supersedes state law.

On the matter of equal protection -- is one cigarette buyer being treated the same as others?

Not really, especially when some big buyers can't be tracked by the state, and others can. But the state can reasonably make the argument that it can't be expected to go after tax scofflaws if it doesn't know about them.

Also, federal law permits progressive taxation legislation -- taxing different citizens at different rates, based on what they can afford -- meaning it's OK to tax someone making $20,000 a year at a different rate than the person making $200,000.

An equal protection claim "is not as strong a challenge," Mr. Ledewitz said. Still, he mused about an 1896 U.S. Supreme Court case, Yick Wo vs. Hopkins, in which a group of Chinese laundry owners challenged a San Francisco ordinance that outlawed laundries built of wood. It just so happened that most Chinese laundries were built of wood, while the handful of laundries owned by white Europeans were made of brick.

The court ruled that even if a law is neutral on its face, if it is enforced in a discriminatory way, it violates the U.S. Constitution's 14th Amendment, the equal protection clause. Justice Stanley Matthews wrote that the division between wood and brick buildings was an "arbitrary line."

Might the 100-carton cutoff also be an arbitrary line?

Mr. Strauss isn't betting on it.

"There's an old principle: Ignorance of the law is no defense," he said. "I think that they're stuck."

But attorney Ira Weiss, who is waging a running battle against Allegheny County's tax valuation system, says the equal protection argument would work better than the uniformity clause.

That's because the constitution is supposed to guarantee not just fair tax law, but also a "fair collection process," said Mr. Weiss. In other words, you can't single out specific taxpayers for collection without specific reasoning.

One hundred cartons is a nice, round number, but if sued, the state would have to demonstrate that there was a sound reason for placing the cutoff at 100 cartons, instead of 90 cartons, or 125. "If an individual believes he's being unfairly singled out," he could certainly sue, Mr. Weiss said.

He pointed to a case in eastern Pennsylvania, Valley Forge Music Fair vs. the local school district. The musical theater venue sued, and won, because it argued that the school district had unfairly, and unconstitutionally, targeted Valley Forge in collecting the local amusement tax.