+1 for having no problems in this scenario. Avis have always honoured it easily. Just have the paperwork as backup. I know you’re going to Colorado but as an FYI a second driver is free in California for all rentals for all companies, however you book.

I also once had a situation in the USA where I had booked an Avis rental directly and wanted to add a US colleague to the rental/insurance as we were doing a longish trip while I was there; we drove back to the airport rental station to ask to add him and they said “if he’s your colleague then he’s automatically covered, off you go”. No paperwork checked or issued, nothing. Seemed very relaxed and even puzzled as to why we had come in to ask!

Additionally if you have time then get an Avis Preferred account setup online with avis.co.uk and get the app and call BAH to get them to add your new preferred number to the rental. They did this for me once after I failed to input it at booking stage. The rental should then appear in your app. You have to put your driving licence and payment card details onto the Avis system as part of setting up the preferred account so they can do the paperwork in advance and charge the deposit but it should mean you don’t need to queue at the counter at some rental stations - you go straight to a board in the car park which tells you where your pre-allocated car is and you get in and go. Even if that doesn’t always work out then you can still use the preferred queue or skip to the front of the main queue.

And further … if you have a priority pass lounge card directly or through Amex gold card you can ask Avis to bump you straight to “Preferred plus” level which gives a free second driver however you book and some other benefits - I did this a few days ago through the messaging portion of the Avis app. Just gave them my Priority Pass card number and it was done in a day or two. I then found out that Mastercard World Elite cardholders get the top level “Preferred Presidents Club” with even more benefits. My wife has the new Barclaycard Avios plus card and I have a supplementary card so I thought I would try it and found an online form where I just had to put name, email, Avis preferred number and what program I was claiming status match/elevation from and selected world elite. No proof was asked for and I am now the top level in Avis preferred. NB even if you don’t have priority pass or world elite cards, note that no proof was asked for 😉. Not sure it will get me a whole lot more as I don’t do that many rentals, but you never know.

Skipping the queue is the best benefit even at base level.