Hopefully this helps you…

In SB billing, for any given invoice, prior to being finalized, you can deselect any time/fee/expense entries from appearing on the on the invoice (as below)….

...thereafter, you can also deselect which entries would be billable (so they won’t show up as “unbilled” in the future).

Finally, at least in our firm, if it’s a flat fee service, we create a new entry with a fixed fee, usually under the primary attorney’s name, tying back to the service description in the retainer agreement.

Hopefully this gets at your question.  Other power users here may have a better solution!