Hello Clara.
I'm so glad to hear that you've upgraded your subscription. I completely understand the importance of customizing your system to work best for your specific needs.
In QuickBooks Online Advanced, the option to add a custom field isn't available for the Product and Services category. However, you can still use custom fields in other categories, such as Customer, Transactions, Suppliers, and Projects, to help you capture and organize additional details. Here's how you can set that up:
- Click on the Settings menu or the Gear icon in your account.
- Under the Lists tab, then find and click on Custom Fields.
- Select the Add Custom Field option.
- Enter the desired name of the custom field, and choose the Data Type.
- You can select the category and select the forms.
- Then hit Save.
Here's a sample screenshot of adding a Custom Field.

For more information on creating custom fields in QuickBooks Online Advanced, please refer to the article: Create and edit custom fields.
Please let us know if you have any further questions. We would be happy to help.