From your Zenmaid dashboard, go to Settings > Job Estimation.

Scroll down until you see the Work Items section of the page.

Click on the +Add Work Item button.

A new blank work item field will appear. Input the work item name you prefer (such as customer name, etc) under the Item field. Key in the fixed rate you'd like as well under the Cost field.

Once done, scroll down until the bottom and hit on the Save Changes button.

Once you've created a work item with the fixed rate you've come up with for a one time appointment, all you have to do now is to add that work item for every same one-time appointment.

From your Zenmaid dashboard, go to you Calendar.

Create the new one-time appointment then search / create a new customer.

On the Create Appointment window, scroll down until the Work Items section is visible.

Tick the checkbox beside the work item you created.

Take note that the field Automatically estimate amount owed should be checked off.

Enter in any additional details for the appointment and click the Create Appointment button.

You will notice that the work item amount has been applied to the one-time appointment.