When I make a bill for an order of the items that I sell, only the quantity auto updates not the new cost of the item.

Hi, ilbsystem.

We'll have to set this up on the Product/Service information, so it automatically shows when you create a Bill. The cost will show under the Rate column. 

Here are the steps:

1. Go to the Sales menu and select Product  and services.
2. Select New.
3. Enter the necessary required fields.
4. Click on Save and close.

I've added screenshots for your visual reference:

Here’s more information on how to enter product and service items you buy and sell: Add product and service items to QuickBooks Online.

Feel free to add a comment below if you have more questions about managing products and services in QuickBooks. I'm always here to help.