Dear webmaster,
since you deleted my message it seems you think that Rick Steves customers should not know that "Cash only" is forbidden by law in Italy.
The moment a company says "We only accept cash" and the bill is over Euro 60 they are infringing the law and they have just proven themselves guilty.
It seems some of the companies you promote on your books do it; whether you know it or not the truth is that you are recommending people who makes money breaking the rules.
Sticking to the OP's question: most probably a guide who takes only cash will offer a flawless service to you and you have nothing to worry about. He's scamming first his honest competitors, than he is scamming me. Not you.
I'd be more worried about getting on a car owned and driven by some guy who blatantly does not respect the rules, but we all decide what risks we are ready to take when we travel in foreign countries.
a VAT number is a line that says PIVA followed by an 11 digit number. Does PIVA have something to do with VAT?
Yes Alan the Italian translation of VAT is IVA,and the vat number is made of 11 digits. "Partita" is an old fashioned way to say "Code" or "Number".
It probably comes from Amatino Manucci, a merchant from Florence who lived in Dante's times who wrote first about a "Double Entry accounting system". May he burn in hell while filing demons' taxes.
if a merchant gives you a hand written receipt on a napkin, that is a fake receipt/invoice.
No, all paper invoices are copies of a digital version these days. It doesn't matter if it's written on an napkin or a towel. What makes an invoice "real" is the digital original and the info written on the paper copy, not the mean used to write those info.
I think You are misunderstanding the obligation to have a cash register for retail stores with the way paper copies of invoices must be issued.