Let me give you some information why the product's cost is 0$, I_dislike_making_up_screen_names.
After you have imported the products from BigCommerce, some of the details was not included. You'll consider checking the Products and Services section to see if the Cost of your product is entered before you sold it because it will reflect in the COGS that its 0$.
Here's how to correct it:
- Go to the Sales tab and choose Products and Services.
- Click Edit on the Inventory item and click the Starting value.

- Enter the Initial quantity and the Initial cost.
- Verify then click Save and close.

After you've change the Starting value and the Quantity of your product, make sure that you've created a purchase transaction.
Also, I’d recommend contacting their Support Team if you're till having this issue after doing the steps above. By doing so, they’ll be able to help you verify the details of your inventory from BigCommerce to QuickBooks Online. Here’s how to reach out to them:
- Go to https://apps.intuit.com/.
- In the Search field, enter BigCommerce for QuickBooks Online.
- Under the App information section, you can get their up-to-date contact information.
Let me know if you have any QBO- related questions. I'm ready to back you up. Take care.