[fierarul]

Timewarrior + a little spreadsheet that gets exported to a PDF.

You are conflating a few things though:

* invoicing per se. You should get the habit of doing this even if the customer only want to know the hours. Paperwork matters.

* invoice details. Try not to go overboard here. Split per project, total number of hours, make a SUM and you are done.

* work status updates. Don't keep the customer guessing. But also, don't allow the customer to manage your every 15minute increment. You are a 3rd party not their employee.