Yours is somewhat simple since TX does not tax personal income. However, it's not "that" simple because if you sell product to someone in another state that imposes a sales tax (and almost all states including TX do) you may (or may not) be required to charge, collect and pay the sales tax to each state you ship sold product to.
While you've taken a great first step at doing your homework "before" finding yourself in a tax hell, you 'really' should seek the knowledge you require from a professional who deals with online commerce and the associated taxes. Remember, there's more than the income tax you have to deal with. When you sell product (as opposed to services in some states) there's a sales tax imposed which has absolutely nothing what-so-ever to do with income taxes.
Some states do impose their sales tax on online commerce, while others do not. This is why you see commercials on TV offering products (those "as seen on TV" commercials) that will sometimes state something like "not available in [name your state here]"
