From my spanish perspective, since it does not show the customers id on the invoice, the Zen Cart "invoice" in the legal sense of the word, does not comply with spanish legislation.
Although my shop is modified to collect and use the customer id, this is just for us to use on the real invoice which is produced by our invoicing software and included with the product. We do not offer the facility to get an invoice online via ZC.
Consequently I have changed all instances of "invoice" to "order details" throughout both sets of language defines to comply with this legality.

It would be (marginally...!) of interest to know how this legally lies in other countries and whether this is a change that should be looked at...ie is the "invoice" produced by stock Zen Cart legal or not in your country?