You can handle this by creating a project and select the 'Billing Method as 'Based on Project Hours'. While there's a service/task which is offered for any give time period. You can log them under a task for the newly created project. Say for example, there are different tasks/services for the project;

• First you add a task for the newly created project.
  • Billing Method : Based on Project Hours
  • Set the hourly rate for the project as (Let's say) $100.00
• Then you can add/log the time entries as 01:00, 02:00, 04:00 hours for three different respective tasks/services.
• Hence the "Unbilled Hours" for the project would be a total of 1 + 2 + 4 = 07:00 hrs
• Once the work is finished, you can create the invoices for the project as either
  • Single line for the project : Invoice amount would be 7 x $100 = $700.00 (or as)
  • Show all time-sheet entries individually : Invoice amount would be calculated with three different items as (1x100) + (2x100) + (4x100) = $700.00

This way you can track the billable hours and can raise invoices based on the billable hours.