If you forgot or neglected to submit a state sales tax exemption certificate to a new vendor and they bill you sales tax on your first invoice, call them to correct their billing system and ask that they issue a corrected invoice before you pay it. Review these invoices and catch the errors before you pay them. You will have a better chance at getting a corrected invoice rather than trying to get a credit from them. The credits normally require some approval down the hall or above them. Reissuing the original invoice normally does not involve others to approve. Prepare a sales tax exemption certificate in their name, send it to their attention ASAP and it will be much easier to get their attention for your request. This paperwork takes them off the hook if they suffer through a state sales audit because you are telling them you are not the ultimate user since the item will be resold.