Can I add a custom surcharge to the subtotal area of the invoice?

Hi there, 

Thanks for joining the Community. When it comes to managing your income and expenses, QuickBooks is the go-to program that helps make your work easier. I'll be happy to provide more information so you're on the right path for achieving your accounting goals. 

The closest feature to what you've described is to manually add service fees to invoices in QB Online. With this feature you can charge customers a service fee for services related to the product or service they're buying. Otherwise, for a feature that does beyond what's described in the article with the highlighted link, I recommend sending as feedback to our engineers. You can send feedback by clicking on the Gear icon > Feedback.

In the meantime, feel free to ask other questions. I'm here to steer you in the right direction.