Can I add the cost to the sales invoice؟

Yes, you can add Cost Of Goods Sold (COGS) to the sales invoice, hakmimalaz-gmail. 

A way to track the Cost of Goods (COGS) is to make sure that there are income and expense details in your inventory items.                               

Here how to create an account to track your cost of goods sold.
 

1. Select the Gear icon at the top, then Chart of Accounts.
2. Click New.
3. Press Cost of Goods Sold from the Account Type drop-down.
4. Tap the closest type of Cost of Goods Sold that matches your situation from the Detail Type drop-down.
5. Hit Save and Close.

If you're using QuickBooks Online Plus, you can use the built-in inventory feature instead of tracking inventory manually. You can refer to this article for further details: Manage inventory. 

However, if you're trying add billable expenses to the invoice, follow the steps outlined in the following: Record and Track Billable Expenses. 

As always, feel free to let me know if you have other questions about QuickBooks Online. I'd be happy to help. Have a great day, and be safe always!