Welcome to the Community, @RitaPeng. I'm here to clarify the cost and sales price in QuickBooks Online (QBO).

You don’t have to calculate the average cost every time manually. QuickBooks Online can automatically update the cost of items based on the latest bill you enter.

Here’s how you can set it up:

1. Enable Inventory Tracking: Ensure the inventory tracking is turned on in your QuickBooks Online settings.
2. Enter Bills Correctly: When you enter a bill, ensure you select the correct item and quantity.
3. Automatic Cost Update: QuickBooks will automatically update the cost of the item based on the latest bill.

This way, QuickBooks will handle the average cost calculation for you, and you won’t need to adjust the item cost each time manually.

For more information, you can visit this article: Set up and track your inventory in QuickBooks Online.

Nevertheless, in case you need to adjust inventory quantity in QuickBooks Online feel free to browse this article for further assistance: Adjust inventory quantity on hand in QuickBooks Online.

If you have other questions about managing your bill transactions, feel free to comment down below. I'll be more than happy to assist.