Hello Piet van Dalen,
Here are my answers for your questions:
We are already having an option to generate invoice in OpenEMR. Are you looking to know the work-flow of it or you mean to ask something else?
To customize the invoice template by adding logo and stuff you need to make some code level changes to some of these files:
/interface/patient_file/front_payment.php
/interface/patient_file/label.php
/library/billing.inc
/library/patient.inc
/ibrary/invoice_summary.inc.php
Thanks,
ViSolve