Sales tax happens at the state and county levels in the US, but as I understand the law here, you are only responsible for collecting sales taxes in states for which you have a presence. Meaning that if you have warehouse in New York and a contact center in California, you would need to charge sales tax for both New York and California, but wouldn't for, say, Wisconsin. Your customers are still responsible for paying the sales tax, they just need to report it on their annual forms.

My best suggestion would be to have someone in each of your locations run to the local corner store, buy a candy bar, and see what the local tax rate is from the sales receipt. That's what you'll need to charge, but again, only from customers who are ordering from a state where you have a presence.

Disclaimer: I am neither a lawyer nor a CPA.