To trigger your recipe when your invoices are sent out to the client, you would need to implement conditional steps on your recipe to filter out invoices which have not yet been sent. To do this, there are 2 fields of interest: 'PrintStatus' and 'EmailStatus'.
If you have not printed out the invoice, the 'PrintStatus' would be either 'NotSet' or 'NeedToPrint'. Once the invoice has been printed, the status will be changed to 'PrintComplete'.
If you have not sent out the invoice via email, the 'EmailStatus' would be either 'NotSet' or 'NeedToSend'. Once the invoice has been sent, the status will be changed to 'EmailSent'.
The conditional action you would need to implement is: "If EmailStatus/PrintStatus is EmailSent/PrintComplete, then continue". The recipe will then execute the following actions after this conditional step only when the invoice has been sent out to the customer. Be careful of the nesting of the conditional steps to make sure you don't force each job to have to meet both criteria in order to be processed.