And now we know who to blame for it.
I'm fairly literal-minded myself, jp, but I would ask before assuming those requests are only suggestions.
The "pony express" shops all say, "the client prefers you pay in cash" but I did ask the scheduler before I used a CC on the C scenario shops. Some people got comments and dings for using credit cards but the shops were still accepted. I got neither commented nor dinged because I said in the report that I had permission to use the CC.
If you think of a "request" like this it might help keep perspective:
If you go in to order and you "request" a cheeseburger with mustard and pickles, and a large order of fries, would it be okay for the clerk to give you a chicken sandwich with ketchup, and a small order of onion rings, without asking if it was okay? If you request a Coke, would it be okay if they gave you a Dr Pepper instead, without asking?
Well, that's how you should consider the client's "requests." You give them what they requested, unless you ask permission to do otherwise.
And if they "require" as opposed to "request" -- don't even ask. If they "prefer" then I would say it's okay to ask, "I know they prefer we get a large soda but I can't fit those 44 ounce cups in my cupholder. Is it okay if I ask for a medium?" But I would ask, not assume it was okay just because they used weaker language in the instructions.
They might accept a variation without rejecting the shop, but you may find they "prefer" the shoppers who follow every criterion exactly.
Time to build a bigger bridge.