PC Service bills for certain typical items through the PMATS interface.  (These are just some I happened to grab from recent PMATS receipts. We can use them to prototype how it can work.)

• MIT0005 PCS Pickup/Delivery @ $35 each
• MIT0003 PCS Labor @ $60 each hour
• PCS Shipping @ $25
• Lenovo 41W5204 T42 Long M10 Fan @ $99.00 each
• Apple 922-8109 Left IO Board @ $64.00
• Apple 922-7956 Battery Cable @ $10.00
• MIT0008 Apple Stock Order Shipping, Apple Stock @ $7.00
• Apple 922-7183 Keyboard, US @ $56.00
• Apple 922-7184 Top Case @ $291.00

That's enough to give the picture.  Pickup/Delivery is a service, as is Labor.  The other items are inventoried.  There would be an inventory receiving discipline in PC Service to add to QBs sense of inventory as parts come in, and remove as they are billed, if removing is automatic.  (is it?  hmmm)

DCAD bills for hours of consulting.  Their item is a labor rate.

DS MDS would bill for ... i don't know, let's thing about them later.