So you want to joke with a cashier
First off, consider why you're doing this. I'm not saying don't do it, I'm just saying you need to know what your expectations are, and consider whether it's a good idea. Second off, don't do so in a way that makes things slow down. They're trying to get through their day, and things that break flow can make that worse. On that note, be conscious of how long the line is; if nobody's waiting, they might be less harried and your humor may land or be served accordingly. Third off, and relatedly, don't joke in a way that you require they acknowledge it. Throw that joke out while sliding the card or while bagging, don't meet their eyes, just say something you think they might find funny and if they like it they like it, and if they don't they don't. (Obviously if you strike out a lot, maybe consider why.) Fourth, don't joke about something that a customer might benefit from, generally, or that would be distressing for a customer to believe or say. "So I guess it's free" is the obvious example and will read as a joke that simply isn't at all funny, but please understand, even if it's known to be a joke, retail workers have to deal with so many people who have such warped expectations all the time. They may have cause to think you're serious. There may be precedent. They don't need that stress. Customers are barely people sometimes.
Fifth, don't take it personally if they don't find it humorous. God only knows what they've been through today. Their brains might be oatmeal and milk and unable to think more today. They have coworkers, bosses, boss's bosses, quotas, and customers and random BS, and it's exhausting. If they laugh, good. If they don't, that's neutral. If they are annoyed, that's feedback. Speaking of feedback, I am open to suggestions, critique, and replies. I've just been putting off writing this for a while. I don't even work retail, I worked in a call center. But I do joke with cashiers a lot because I want to improve their day, and I wanted to talk about how to go about it and the mindset to have going in and coming out of it.