Hi, i m serious freelancer with 100% completion rate and i believe i can do this for you. As far as i see, you will have only first sheet "Invoice data" and from that sheet two other sheets are created (Products and Purchase order) after those 2 sheets are created they are exported as csv (like in your example in the last sheet). (Generated csv, not using save-as, so you can get results exactly as you want them).This is really simple work it will take me around 2h (max), probably less then that. So please send me a message.
Edit: "I m not sure if you already chose a freelancer, but i created a demo for you that works exactly as you specified, this is interesting project for me so i had a little fun doing it, so if you are interested please pm me and i will send you a demo to check."
Thanks, Milan