The distinction between "supply" and "material item" is really not clear in either of these resources.

Scope and Usage
Transport
"The Transport resource captures the requested movement and movement history of persons and items in the healthcare spectrum. The Transport resource documents the movement of a person or item.
...
Sample use cases for the laboratory are as follows:
Movement of a specimen between distinct healthcare locations (clinic to lab, lab to lab, lab to biorepository) without the use of a third-party transport system (e.g. a courier)"

SupplyDelivery
"The SupplyDelivery is used to document actual movement of supplies (e.g., medications, devices, substances, biologically derived products) from one location to another, where the intended patient or person (e.g., doctor) may also be known."

It would help to see wording on both that indicate why they are different than the other resource. They are both moving items, which may be supplies or specimens. One is initiated as a SupplyRequest one is initiated via ServiceRequest or Task. The distinction between the two is not very clear.