Here, we are connecting Order with Product. To associate the two, we only need the IDs:

• Product ID
• Order ID
• Quantity
• Item amount (individual value)
• Tax amount (individual value)
• Total amount

We are saving the amount of one individual item as well as the tax for this one item. The total amount then is as follows:

total amount = quantity * (item amount + tax amount)

The total amount of Order will then be the sum of all Order Product IDs.