You can assign a fixed amount per month to bill the clients in a particular rate group and allocate the tasks that are covered by this fixed amount.
Fixed Retainers are set up in the Rate Group table under Setup by clicking on +Record in the top right corner.
Name the rate group, and add an amount under Fixed Fee for Included Services.
Click Save & Edit
Then click on the Included Services tab at the top,
Add tasks that are part of this fixed retainer by clicking on +Record.
The invoice process will then only invoice the client the fixed amount per month, whether 1 minute, or 40 hours were booked, even if you don’t use the timer at all.
If your fixed fee package includes services that aren't included in the fixed amount but billed for separately or hourly, you can further expand on the rates by adding these tasks under the rates table.