I was able to make it work but unfortunately, it is generic. I can only choose posters, which are $11.99 or posters + tickets, which are $19.99. When someone orders a poster, it is a specific poster, and going to the independant non hosted discount page can only calculate quantities of posters to provide the discount, which is not good as my client needs to know the exact poster. Is there a way to do it according to sku's? If the customer knew the sku it could possibly work. How am I able to put the sku visible on the shopping cart?

I think you want this to do more then what it's intended. It's a simple discount and taking it beyond that isn't so simple and would require a bunch of custom work.