E-Commerce Terminology and Formulas

I can't seem to find straight answers in my Google searches (perhaps I'm not searching for the correct keywords?) and need some guidance. What are the values in a standard e-commerce system and how are they calculated? For example in a single order containing multiple products...

Gross Sales - Is this the sum of the retail product prices before any discounts? Or is this after discounts? Is this term the same as a Sub Total?

Gross Revenue - Is this the Gross Sales minus discounts? What is the difference between Gross Sales and Gross Revenue?

Where does Sales Tax and Shipping Charges play into these numbers? In some fashion, the online payment will be the price of the product + sales tax + shipping costs so there must be a standard metric that describes the total money collected?

Does anybody know where I can find a good resource for this information or have time to explain what these terms mean in an e-commerce context and how to calculate them?