You do not have to collect Texas tax on items that you export, but you must keep records to prove export.
The Comptroller’s office will accept any of the following documentation as proof:
Yes. Texas sales tax is due when your customer picks up or takes delivery of a taxable item in Texas.
You can refund the tax collected to your customer if they give you acceptable proof of export certifying that the merchandise was taken outside the country. You must keep a copy of the export certification for four years as proof that the refund of sales tax was for an exported item.