Im from Nevada.

Recently, I had a customer who was from Alaska, come into my retail store and bought some jeans. I told her the total price, and she refused to pay the total price, because she said that she was from Alaska(NON PAYING TAX STATE) and that ,ACCORDING TO FEDERAL LAW, she didn't have to pay the taxes IN NEVADA.

Nevada has its law stating that no matter where you come from, you have to abide to the tax laws...

Was my customer right?
What federal law states that she didn't have to pay taxes?

Not really. IF she had asked you to ship the jeans to her in Alaska, then you could have legally not collected the sales tax. But if she was taking the jeans with her, she needed to pay the local tax.

Agreed. That argument has been argued in federal tax court, and defeated.

The states have the sovereign right to tax whomever is within their state borders, be they visitor or resident. That applies for sales made within the state or income earned within the state (for state icnome taxes).