The customer sends you payment and a registration form. Then, the shareware is transferred by E-mailing the customer a registration code that unlocks the software. As the registration agent for the author who lives in Australia, you keep half of the registration fee and send the author the other half.
Software including shareware is considered tangible property and is taxed accordingly.
Texas sales tax is due when an individual downloads software from a Texas server. Texas use tax is due when a Texas resident downloads software from a server located outside Texas. Texas is a member of the multistate compact and will allow as a credit against Texas use tax due any combined amounts of legally imposed sales or use taxes paid on the same property to another state or any subdivision of another state. Credit will be allowed even though the other state may not be a member of the multistate compact. You should collect tax on the total charge for the software, based on your location in Garland.