The Estimation using Inventory and PriceBooks can both be switched on at the same time at the Company Settings page. You will be creating estimates using details from both your Inventory and Pricebook including prices. Both your Inventory and PriceBooks will have to be set up for this to work properly.
Both the PriceBooks and Parts & Services fields will come up on your estimate.
Filling in both the PriceBooks and Parts & Service fields is not required. You can fill in only one of the two.
As you add items to your estimate, the Subtotal field at the bottom and the Estimation Summary at the top will auto-calculate.
Once done, you can then proceed to add/set up the discount and taxes and complete the estimate.