Mexican citizens are not exempt from sales tax when buying merchandise in the U.S.
The only way we can sell a Mexican citizen without sales tax is to send them their merchandise through an export company. We have to charge the sales tax up front, deliver the merchandise to the export company, get a bill of landing from them, wait until delivery is complete, get a copy of the importation documents from them, then refund the sales tax to the customer. The customer may have to pay an import duty to the customs in Mexico as well.
We can not deliver merchandise to Mexico ourselves. We are not licensed to do business in Mexico, not to export.
As it is quite a process to export, it is best to just include sales tax in your price quote and back it out when writing up the sale.