If my customer pays the use tax, why do I have to charge him tax. Can't I get a letter from him that says he'll take care of it?
The answer is, generally, NO! The state doesn't trust your customer (and with good reason, I might add) to pay the use taxes. You're the seller and you are primarily responsible to collect the sales (or use) taxes from the customers. Even if they "promise" they'll pay.
Most states have a "direct pay permit" which DOES get you off the hook. This is a state-sanctioned way for a customer to tell you, "we'll take care of the use tax ourselves." The customer has to apply for the permit, and they will generally go through more audits than the rest of us. That's because the state really DOESN'T TRUST anyone to pay their use taxes.
Bottom line, you have to collect the taxes from your customer.
Sales Tax Guy