If there were three classes of sales tax (essentials/normal/luxury), everything would have to belong to sub-classes. For example, medical supplies could be a sub-class of essential items and would not be taxed. Clothing (all clothing, but possibly under a certain price) could be a sub-class of normally taxed items. Yachts or cars over $30k (where I would make the cut) could be sub-classes of luxury items and be taxed at a higher-than-normal rate.
In summary, it could be set up such that if you bought a pair of $100 jeans, you'd pay normal tax, but if you bought a pair of $1000 jeans, you'd pay a luxury tax rate. Of course, it could be set up completely differently. But remember, one of the benefits of this type of system is simplicity. If you make too many divisions, you lose that benefit.