Hello, Aam274.
I have a way to help you calculate the shop supplies on the labor for your mechanic shop.
You'll want to add a subtotal to your shop supplies, then add the labor service to calculate the 6% charge.
First, create a subtotal item:
- Go to Lists, then select Item List.
- Right-click on the Item List window, then select New.
- Set the Type as Subtotal, then add a name for it.
- Once done, click OK.

If you haven't already done it yet, you'll also want to create a labor service as an item. Set the Type as Service, then enter 6% in the Rate box.

When creating an invoice, you'll want to add the shop supplies first, then add the subtotal item.
Afterwards, use the labor service right below the Subtotal line. This will calculate the total price of the shop supplies plus the 6% labor charge.
Add the remaining items for your invoice (the parts in this case). This is what it looks like after creating the transaction:

If you've noticed, the labor service item will always calculate the prior item's price.
Need to see how much you've made from your mechanic shop? You can check out a bunch of reports to see your sales, expenses and other data.
This article can help you build your preferred report: Understand reports.
I'm happy to show you around in QuickBooks Desktop again. If you have more questions about recording invoices, please let me know. I'd be glad to help.