Product knowledge is a HUGE deal. You can increase your income quickly with product knowledge alone. Upselling retail to clients, creating custom packages that cater to client needs, and using the right products for the right hair textures and goals are vitally important.
A working spray isn't the same as holding spray. A serum isn't the same as a shine.
Study your products. Understand what's in them. The more you know, the easier it is to cocktail. Test and try different variations to cater to clients needs. How well do yoy know your products?