Time and material can work but it all depends on how you lay out your invoice. We have a service line where we describe what the problem was and what was done, as well as any PO information and the requestor.
Next we have our labour line which identifies the tech and rate, then truck charge, and finally materials (the description lists the material but its one line item with a total cost not a cost for every item used, I caution against this for price checking clients), after materials we can sometimes have permit or parking charges.
The biggest help with T&M jobs for us was the development of photos in job forms, with every invoice we send a job form stating who went what we did and any findings or observations that were made on-site as well as material that was consumed.
This has really helped with the blow back from clients, hard evidence makes it hard to argue. Some of our clients have made it mandatory for other trades to provide similar reports to justify costs.
We use chatGPT projects with specific instructions to take our technician notes and craft these reports.
I hope this helps!
