Tax is based on the shipping address of your order. As mandated, we are required to collect tax in states where we have "nexus". Here is a brief explanation of the law, and I hope this information helps.
"If your business has a physical presence in a state, such as a store, office or warehouse, you must collect applicable state and local sales tax from your customers. If you do not have a presence in a particular state, you are not required to collect sales taxes. In legal terms, his physical presence is known as nexus."
Source: U.S. Small Business Administration
Note: A physical presence can also include residence of an employee.
Please let us know if we can be of further assistance, and we appreciate your understanding.