"We have ONLY used QuickBooks for creating invoices, nothing more."
Then, I would treat this as a Transition task, as if you are New to QB. Start a New Data File.
"
I
spoke with QuickBooks on the phone and they recommended using
Non-Inventory Part for our mattresses if we turn inventory off. So is
there a compelling reason to use Service Item instead?"
Use Noninventory for Materials.
"A
service item by default posts to income."
That's not true. There is no Default. The assumption is that there are things you buy or sell or buy and sell; but any item is linkable to nearly any account.
The Reason to use different Item Types is for Better Reporting.
Service Items for labor or design or other intangibles.
Noninventory for Materials goods.
Other Charge Item Type for Freight or Restocking Fees.
Then, your Reporting gives you Segregations.
Examples are attached.
