I've not designed a point of sale before and frankly, don't have much knowledge of what attributes are needed. I found the following schema and realized that it still won't store everything I need; I'm still missing some attributes.

Some of the issues that I see with this digram:

1. I need to allow for product returns.
2. I need to allow no charge items.
3. There should be an allowable discount for each product and also an additional discount that can be applied at check out.
4. The pos_invoice table doesn't have a total attribute.
5. How to handle voided transactions...

I think I can add a total column to the invoice table but I'm wondering why it wasn't there to begin with. So where should these attributes go?