You can invoice a job based on the time you spend on it thanks to the time tracking functionality in LSP.expert.
For this, you have to select the "Time-based invoicing" option for the job concerned, in the Invoicing type field in the billing panel of the job. You can do that when creating the job, before beginning to work on it, or afterwards.
Then, you can start working on the job by using the time tracking functionality (see this article to know how to use this functionality: https://help.lsp.expert/en/articles/728072-how-does-the-time-tracking-feature-work).
Once you finished the job, you can see that all your time entries in the Time tracking tab of the job are marked as Billable.
If you started tracking your time for the job before selecting the "Time-based invoicing" option, you can go and manually mark your existing time entries as billable in the time-tracking tab of the job after you selected the "Time-based invoicing" option in the billing details of the job.
The system will add up all the billable entries and this total time will appear on your invoice (you can modify it manually in the invoice if you want).