As it stands now, Texas has no authority to negotiate tariff rates while within the union. In fact, the vast majority of current trade agreements negotiated by the federal government are beneficial to a handful of economic sectors within Texas and detrimental to the rest.
Texas will negotiate both bilateral and multilateral trade deals with other countries. Those trade agreements will govern the terms with cross border transactions. At a minimum Texas can engage in trade under existing World Trade Organization tariff schedules. Both WTO tariff schedules and negotiated trade agreements set the terms under which tariffs are laid for all goods and services as well as the amount of those tariffs.
In short, unlike our present situation within the union, Texas will be able to negotiate trade terms that are exclusively beneficial to Texas.