Personally I would just wait till you see her again in store. I wouldn't visit her like the next day. Just wait a week. And catch her at a time the store isn't busy.
Next, hit her with a DHV story that gets her emotions rolling. If you think about it, cashiers are stuck working their boring 9-5 jobs every day. They want something to pull them out. Something to not bore them as much as "Hey how are you?" "That's a lot of toilet paper" "Yep." "Have a nice day".
So I would tell her this story about how you hit a deer with your car.. and really sell it to her.. "it was raining late the other day.. and I was leaving my grandmas house... I took her to visit grandpas grave one last time.. it was his birthday.. and out of know where.. this poor little deer just jumped out right in front of my car! I hit him and sent him flying.. I went out to see if her was ok but he was just.. shaking and trembling on the road.. there was blood everywhere.. he was hardly moving.. and I just felt so bad for him.. I knew I had to put him out of his misery.. and then... (make twisting movements real fast with your hands) I snapped his neck!.. I knew there was nothing else I could do.. since he was already so close to death... but other than that my car is fine and I got it all fixed up.. I just.. couldn't help but feel sorry for the little guy

"
You could even build up the drama more by saying "and after that.. on the side of the road.. I saw the rest of the deers family just staring at me.. I can't really tell you how I felt.. (just stare somewhere lol) seeing what I saw.. really sticks with you.. you know.."
At this point she should be like "Yeah..." Looking deep in your eyes or something - if you really played it off well. Then tell her "I'm just fucking with you. That never happened. Hahaha." At this point she should be calling you a dick and text you "I fucking hate you!" Or something along those lines. After that wait two days and start texting her again. I really need to start blogging lol.