Time on the job. I used to have a flat fee for supplies before the job.

If I bid a job by the hour, any time needed for supplies not planned for was included in the bill.

Contractors (as well as any other professional) charge you from the time they leave their house until the time they return thereto.

Time spent picking up materials, ordering materials, placing materials, consulting with other professionals, administering the project or otherwise making sure your job gets done is part of the total and overall cost...and should be.

Yes, getting materials is chargeable.

I might quibble if they forgot something they knew they should have had on hand.