Sales Tax

[ERSpangler]

I'm creating a site for a customer selling dog food (my first with any sort of shopping cart). The customer requested a Paypal cart and I found it very simple to set up. The customer also asked me to include sales tax on her products. Now, how do I determine that? Does everyone who purchases an item from her pay Washington State sales tax? Only those from Washington State? Only states that pay sales tax, thus leaving out Delaware, Montana, Alaska and a few others?

Thanks!

Erin