Thanks for getting back, kim-z. 

QB works base on the current transaction created. That is why COGS would change every time you make a transaction. However, you can manually calculate the changes from the first to the recent transaction. 

Here's how:

1. Click the Gear icon and under the Tools section, choose Audit Log.
2. To easily locate for the invoice transaction you can filter the report. 
3. Choose the Date (From and To) then select these options: 
   •    Events:  Show only these events
   •    Select Transactions
   •    Show: All Transactions
4. Then click Apply. 
5. Under the HISTORY column, click the View link. 
6. From there you can look into the changes of the Qty and Rate. 

 I recommend submitting feedback about the COGS function.

The Community is around to help you if you have additional questions.