How does Zazzle know which states to collect tax from on my behalf?

Zazzle collects tax on your behalf from the states and territories that you direct us to do so, according to the Shipping and Sales Tax settings in your account.  

You can manage your sales tax settings by logging into your account and going to the Shipping and Sales Tax tab in your account settings. To select which state you wish to collect tax from, simply click on the box next to each of the relevant states and click save. There is also an option if you do not collect sales tax.

Once collected, Zazzle will forward the full sales tax amount to you during each bi-monthly pay period.