I have provided you my US Treasury §501(c) document. Why are you still charging me tax?

Status under §501(c) allows for exemption from US federal income tax. The tax INSPIRE charges on its invoices is neither a federal tax nor an income tax but is instead state (or other local) sales tax. As such, the states will generally require exemption documents specific to their state. While certain states provide eligibility for certain organizations exempt from federal income tax under §501(c), those organizations need to apply for and obtain proof of exemption from that state.