I see two points here that I think I covered before but people are discussing it still.
First, business policy. It's common knowledge that you need at least an open water card to dive so shops don't normally tell you to bring it. However, if they have other required info, they should tell you what it is so you bring it. They were fully prepared to make me waste an hour of my time, but not willing to take 5 seconds to tell us on the phone of the requirement.
I do think it is highly unusual to require a nitrox card. Granted, I've only been doing this for 7 years. I've dove lots of places, boats and gone to lots of shops in multiple continents--but I have never had to show a nitrox card. Even for a fill. It literally is a card I have never shown to a single operator, tank monkey, or clerk. Never.
What is more common is to ask for an AOW card, and my DM card covers that.
The shop told me "it's Sunday morning no agencies are open so try to look it up yourself or go home and get it." Not exactly helpful.
The second point is, why not carry it? Simplest answer is between myself, significant other and friends, we take different cars from time to time. I keep with me only the two cards that I have ever needed: DM and full cave. Like I said, I've never needed anything else, so why carry it? I keep them in my wallet, a very handy place. I keep my wallet in my pocket, so I try to keep it slim. Credit card, ID, insurance for car and diving...it adds up. I have a pile of cards, I couldn't carry them all. And, I have never been one to take a logbook with me. I call before I to to places and if they don't tel me to then I don't bring it. Same reason I don't take my drysuit and my wetsuit...at some point it gets ridiculous.
I do keep photos of cards in my phone, but apparently that one never made it in there-my fault but again--6 years, the card has never been shown to anyone. Funny thing is, I had it in the car after all.
Really it's quite simple: the shop didnt tell me what they wanted, I brought what 7 years of diving has shown me is typically needed, and the shop treated me like the idiot. I won't go back to a dive shop that scolds me for not doing something I was not told to do.
I won't go back to the shop that couldn't figure out how to fill doubles, or the shop that requires students to buy $7 mouthpieces for rental regs. Now I have 3 shops I won't go to. I'm sure all 3 have loyal customers, and I'm sure they will all do just fine without me, and I'm ok with that. I'm not vindictive, I just only give what little money I do give to shops that treat me awesome and go above and beyond to please me. For example, Divers Direct. I had a small annoying experience with them years ago where they refused to fill a tank because it was too big for their little bomb shelter. I gave a different location some small business-a few tank fills-this year. Apparently I forgot to make it clear I wanted nitrox. They immediately fixed it without complaining at all, accepted my MOD stickers as proof they were nitrox tanks, didnt complain about the VIP stickers being on the bottom of the tanks, and didnt hassle me about a nitrox card, plus the tanks were filled to 3200psi when relatively room temperature. Then they gave me info on a local dive site and what competitor of theirs was closest to that site and could help me if I needed more details. I'm impressed enough that now I've been getting more and more fills from them. Still not as good service as you get in cave country, but they served me as if I were the most important person in the building at the moment.
I will get the nitrox card into my phone, but I still don't see why it was needed, and their customer service, while far from abysmal, did not warrant a repeat visit. If others are happy there then by all means it might be a nice place. They had a wide selection of masks and fins and spearguns and stuff, very well stocked and organized. It was a nice operation until they started enforcing poorly thought out and communicated rules that are far from the standard in diving.