I appreciate you providing an in-depth detail of your concern about adding columns to your invoice template, Matteo_S. I'm delighted to address it by providing an alternative way to achieve it.
You're correct that the custom field feature will not be included in the table section of the invoice, as it will create an additional field in the upper part instead.
The option to add a column in the table section of the invoice is currently unavailable. At this time, we can utilize the Item Description column instead. However, this can only apply if you have no product descriptions.
To guide you further, please follow the steps below:
- Go to Settings ⚙ and then select Custom form styles.
- Click Edit in the form style you're using.
- From the Content tab, pick the Table section.

- Tap Edit labels and widths, and change the Description column name.
- Hold the box beside the Description column and drag it to where you want it to display.
- Hit Done to save the customization.

Once done, leave the Description column blank and select the correct template in your invoices. So it will appear like the screenshot below:

While this feature is unavailable, I encourage you to submit feedback requests to our product developers. Rest assured that they will review and consider it for future updates, depending on the number of users' requests.
Additionally, we have built-in reports that you can use to see and manage your inventories in QBO. Check out this article that explains what you can see when you run the specific report: Use reports to see your sales and inventory status.
If you have further questions about managing your invoices in QBO, just let me know by leaving a comment below. I'm always here to help.