You do not have to collect Texas tax on goods you shipped directly to your customer outside of Texas, but you need to keep documentation of the interstate shipment. Examples of documentation include trip tickets, truck log records, freight forwarder's receipts, and airway, ocean and railroad bills of lading.
You do not have to collect Texas tax on goods that you export, but you must keep records to prove export. The Comptroller will only accept one of the following from the seller to substantiate a claim of export:
Yes. Texas sales tax is due when the customer picks up or takes delivery of a taxable item in Texas.
Once the customer exports that item to another country, you can refund the tax collected if the customer gives you acceptable proof of export certifying that the merchandise was taken outside the United States. You must keep a copy of the export certification for four years to substantiate that the refund of sales tax was for export.