I solved nearly all my problems. Last thing I'd need help with, is an image that needs to be at the bottom of every page. Unfortunately, every way I can think of turns out to be the wrong way. Most of the time, it shows on the next page, which means I'd need two pages for every invoice. I tried it with a page footer as well, which lead to the same result. I'm stuck here.

For those who have the same problems I talked about in the first post:

I put a list tablix over the whole page and then used several tables (all grouped by invoice number) to create the layout I needed. It worked like a charm.