How can I create multiple labels for the Quantity value in an invoice template?

How can I create multiple labels for the Quantity value in an invoice template?

I'm trying to bill for services by the hour plus add expenses by the each. But I cannot figure out how to customize the way the items are described. 

The services are saved as a project with time entry and set to bill by the hour. I add the project to an invoice using my template. In my template, I set the label for "Quantity" to say "Hours" at the top. That part works as I'd expect.

But I also have unbilled expenses that I add to the invoice. These expenses are categorized and given a dollar amount. They are not hours; they are tangible costs. I don't need a quantity value at all, certainly not one that says "hours"!

When I add an expense to an invoice, it automatically inherits the labels from the invoice template. I'm asking for reimbursement of an expense incurred on behalf of the client with a single transactional total. There must be a way to delineate project data and expense data so they don't inherit the same labels.

Is this a setting in the template, in my preferences for custom labels, or in the expenses? It seems like a fundamental idea, so I'm hoping I'm just missing something.