There are different types of respect.
- Deference to a person who is in a position of power over you, be they manager or police or some other representative of the justice system has nothing to do with what a person does, or what they are like as a person. It is entirely about the position they hold, be it manager, police officer or representative of the justice system, teacher, etc.
- Earned respect is a different beast. It has nothing to do with the position a person holds, and everything to do with the way a person behaves.
- Basic human respect is again something else entirely. It's the respect you get because you're a human being with rights. It has nothing to do with the position you hold or the things you've done, like the other two kinds of respect.
What we have in the usual staff/customer interaction is a discrepency in position/power. The fact of the matter is, by virtue of being the employee, the staff member is already at a disadvantage. Not only must we adhere to the social contract, but we also have the rules and regulations and policies of our respective companies to follow - many of which you as members of the public are not privy to. And many of which you as members of the public exploit mercilessly.
Nice customers understand this power imbalance and do their best not to exploit it. They see us as equals.
Crappy customers understand this power imbalance and exploit it for their own ends. They see us as less-than.
The OP was a crappy customer. She didn't give a toss about what the staff member was telling her. The staff member, by OP's own admission, used the word "can't", as in, she can't get the cucumber because of the policies and restrictions she mentioned. The OP then wilfully misinterpreted that as "won't", as in, she won't get the cucumber because she's lazy and rude.
The OP was not wrong for asking the question, she was wrong for not accepting that the answer was no.
The staff member was not wrong for being exaperated with OP's rudeness, but she was wrong for expressing that verbally while on duty in a customer facing role - IF we take OP's word at face value and believe that the staff member spoke rudely when delivering the final remark. Given OP's already demonstrated deficiency at auditory comprehension, I'm willing to bed that the staff member wasn't rude - OP just thinks she is because she was saying something OP didn't want to hear (and not immediately jumping to comply with OP's unreasonable request).