Mallory..
Since you are an operating business you should have a tax permit, which will give you a tax id. This is the 'legal' way of running a business in Texas. A tax permit is super easy to get, just call your local tax office for help. For online sells you only have to charge taxes on Texas customers. At the end of each month you will pay in your taxes off of those sells.
If you sell at a show, some shows require a tax permit, others don't, however you should already have one anyways since you sell online.
I'm doing a show in Sept. and asked if I needed to bring my permit with me and they said they don't check for them....but each show is going to be different and if the IRS finds out you are running a business without your permits then that's not going to be fun.
you can register for a permit online using this link....you might have to get a DBA as well, something else to look into. If you need a DBA then you have to get that first before you can get your tax permit. In my case, I had to get a DBA first.
http://www.window.state.tx.us/taxpermit/