Tex. Tax Code § 151.307

(a) Tangible personal property or service that this state is prohibited from taxing by the law of the United States, the United States Constitution, or the Constitution of Texas is exempted from the taxes imposed by this chapter.

(b) When an exemption is claimed because tangible personal property is exported beyond the territorial limits of the United States, proof of export may be shown only by:

(1) a bill of lading issued by a licensed and certificated carrier of persons or property showing the seller as consignor, the buyer as consignee, and a delivery point outside the territorial limits of the United States;

(2) documentation:

(A) provided by a United States Customs Broker licensed by the comptroller under Section 151.157;

(B) certifying that delivery was made to a point outside the territorial limits of the United States;

(C) that includes, in addition to any other information required by the comptroller, a statement signed by the person claiming the exemption that states that "Providing false information to a customs broker is a Class B misdemeanor."; and

(D) to which a stamp issued under Section 151.158 is affixed in the manner required by that section or Section 151.157;

(3) import documents from the country of destination showing that the property was imported into a country other than the United States;

(4) an original airway, ocean, or railroad bill of lading and a forwarder's receipt if an air, ocean, or rail freight forwarder takes possession of the property; or

(5) any other manner provided by the comptroller for an enterprise authorized to make tax-free purchases under Section 151.156.

(c) Documentation, including the stamp affixed to the documentation, that is provided by a customs broker licensed by the comptroller under Section 151.157 is presumed valid in the absence of clear and convincing evidence that the tangible personal property covered by the documentation was not exported outside the territorial limits of the United States.

(d) A retailer who receives documentation under Subsection (b)(2) relating to the purchase of tangible personal property exported beyond the limits of the United States may not refund the tax paid under this chapter on that purchase before:

(1) the 24th hour after the hour stated as the time of export on the documentation, if the retailer is located in a county that borders the United Mexican States; or

(2) the seventh day after the day stated as the date of export on the documentation, if the retailer is located in a county that does not border the United Mexican States.

(e) A retailer who makes a refund before the time prescribed by Subsection (d) or makes a refund that is undocumented or improperly documented is liable for the amount of the tax refunded with interest.

(f) In this section:

(1) "Air forwarder" means a licensed International Air Transportation Association freight forwarder.

(2) "Ocean forwarder" means a licensed Federal Maritime Commission freight forwarder.