I really like the way that you present your road map, I will implement this in the future.
Being that this POS is meant to be universal it would be nice if you set up the sales tax to be reliant on different states and also withing the states there are tax agencies that require they get their cut which all have their own rules on what type of inventory products are taxable.
This is the reason I use a code in my product table which can be used to identify different types of products such as clothing, jewelry, tires, food etc.
Then the tax tables for each of the tax agencies can determine what the tax rate is for each produt.
This is very important for those of us that sell online with the new nexus rulings for sales tax.
I know this is more involve than you intended but this is a very important part of today's business model's. Most of the students here are likely small shops that sell products online.