My boss has been unreasonably abusive since I have handed in my resignation

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Through great shock and delight I obtained my dream job as an admin assistant after working in garages, as a door to door sales person and in clothes shops. My direct supervisor was sad to see me go but was happy for me as where the other staff that I work directly with. I work in a large garage/ supermarket. My boss on the other hand however had it out for me since she saw the notice, I was busy refilling the empty cigarette cabinet and serving customers at the till at the same time when she announced that I also need to put the newspapers together which on a Saturday is a big job, the overflowing charity box had also collapsed and spilled everywhere which I was trying to fix. I calmly explained that it may take me a while to do the papers since I was already trying to do 3 jobs and I was the only person on the tills. She went off in a huff but I really did try to get some papers done before it got busy but it was impossible to do everything myself. Then the till system started messing up fuel that was being put into cars wasn't coming up on the screen and the till system wasn't accepting card payments which meant everything had to be typed into the machine, I accidentally put through a payment before the customer had given me their last item which meant they couldn't get their 6p/c off their petrol. Unfortunately since I am just a sales person I cannot operate refunds and I really wanted to offer good customer service so the first person I saw was the boss putting biscuits out on the shelf so I asked her help explained that it was my fault that it happened and I was sorry to both the customer and too. She told me I needed to refund and put the sale through a second time expecting me to know how that was done of course I didnt and she yelled at me in front of about 20 customers and 4 other staff members. I really thought that was appropriate and only made her and the company look bad as I was only trying my best in a bad situation and the customers saw that. So why the hell should she take it out on me, maybe if she was nicer then fewer people would be leaving work to find alternative employment. I appreciate that she is stressed that she needs to find another employee to take my position but she could have done it in private as her behavior dose not help anyone. Has this happened to anyone else? should I be mean to her back and not turn up for my last week of work as revenge? Should I send her something comical in the post with a message saying chill out? Or should I even report some of her other activities to the head of the company as this isn't her only mistake she has made many that I have researched and could very well be illegal.If I am to be humiliated for making a minor mistake that can be fixed easily should she not be punished for ones that could very well be illegal?