The reason your salad was taxable is that it was prepared. In Texas (and probably most other places), prepared food is taxable. If you buy the individual ingredients, it's not.
The Texas state sales and use tax rate is 6.25 percent, but local
taxing jurisdictions (cities, counties, special-purpose districts and
transit authorities) also may impose sales and use tax up to 2 percent
for a total maximum combined rate of 8.25 percent.