Of all the different types of Karens, the customer service, "let me speak to the manager" types are my favorites simply for how spectacularly ineffective they are. Just like toddlers, screaming won't get you what you want, and neither will flopping down on the floor.
The key is to keep it friendly and smiling, ask continuous questions and, most importantly, never refer to the company as " you" when speaking to an employee. That sets up an "us vs them" scenario that puts the customer at odds with the employee/ business.
I've found it works best when talking to an employee to call the company "them." It puts the employee on your side and sets up a subtle "us vs them" with the company on the outside and the employee and customer on the same side.
Example: "I understand that they want to keep loss at a minimum, but I don't feel comfortable leaving my bag from the other store up front with you. I mean, that leaves you responsible if something happens! I wouldn't want to do that to you! Do you have a stapler, maybe? Could you just staple it shut? Or, I could put the whole thing in my purse? Thank you so much!"
Works much better than: "You think I'm going to steal from you? You say that you love your customers, but you obviously don't. I'm not giving you my damn bag. I want to talk to your manager! And then you will honor this coupon!!"