If you're shopping online, not only do you need to keep track of your purchases, you need to keep track of your sales tax.

Granted, it's a total holiday buzzkill. But if you're constantly complaining about the beat-up roads and poor schools in your state, then tally up and pay up.

Here's why.

A sales tax is basically a consumption tax on the goods and services you buy.

It's sent to your state and used to fix those roads and hire better teachers.

Now this includes your online purchases too, but some online sellers collect sales tax and others don't. 

Online sellers that have a physical presence in your state have to collect sales tax, so if you order something from Macy's and there is a store in your state, they're supposed to charge you sales tax and send it to your state.

But what if the site doesn't collect any tax from you?

In theory, you're supposed to keep track of all purchases you made that don't have a sales tax attached.

Then when you go file your state tax return, you will pay the tax. It's called a use tax when you file because you actually made the purchase out of state.

It's semantics and it's really not that much money.

Many sites now put up a consumer warning that says, "You are liable for use tax if you are not charged a sales tax at the time of purchase."

Now, there actually are a bunch of states that don't even have a sales tax: Alaska. Delaware, Montana, New Hampshire and Oregon.

Remember -- what matters is where you live. So if you live and shop online in Oregon -- but send a sweater to your aunt in Connecticut (which has sales tax), your aunt, in theory, will owe a use tax when she wears that sweater to church on Sunday.

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