I will be following this topic - same goes for me - cannot use the GURU invoice function as is.
Workaround will be using this as "proforma invoice" - and afterwards sending a "real" invoice regarding to law.
Not really pro to the customer - not preferable to spend time on it either...
Another issue is, that countries often have different VAT percentages - this, ideally, should be handled in backend so that a customer pays the VAT that applies this his/her country, based on the country the customer have entered in the registration form.
Thanks for bringing it up.