There are two ways to invoice once you have integrated ServiceM8 with MYOB.
Once a job has been completed and is ready for invoicing, you mark the job as approved (via the ServiceM8 invoicing page). This will transfer the invoice details including all items/services into MYOB as an invoice ready to be printed/emailed from MYOB as per how you would use MYOB without ServiceM8.
The advantages of this method is that you are able to continue using your MYOB invoice template, and accounting staff do not need to change their current process while allowing the rest of the business to use ServiceM8.
Once a job is completed and ready for invoicing, the invoice can be produced via ServiceM8 and emailed/printed and sent to client. Once the job is approved (via the ServiceM8 invoicing page), the job / invoice details will be transferred to MYOB for accounting / payment chasing purposes. This approach requires that you no longer use your MYOB invoice template, but switch to a ServiceM8 Invoice template. You can still use MYOB to issue statements, etc - however your main method of invoicing clients is done via ServiceM8.
The advantage of this method is that staff are able to issue professional invoices to clients (as required) without having to have access to your accounts (and even via their iPhone/iPad). It's also much quicker to produce and email an invoice to a client from ServiceM8 due having a pre-typed email for invoices. Invoices produced or emailed this way will also be stored in the Job Diary for future reference. ServiceM8 invoices can also do things that MYOB invoices can't - such as adding the client signature to the invoice as confirmation they approved the work as completed.
Finally, if you would like to mix-and-match between some jobs being invoiced via ServiceM8, and some via MYOB - this can be done as well, all you will need to do is ensure you setup a similar invoice template both in MYOB and in ServiceM8.