There are many solutions to find for something you would expect to be kind of default functionality. But no, no default, rahter painfull when you are figuring out you selve. In my opinion, the setup is not logic. If you try the logic way, you'll get strange and unexpected behaviour.
So the one solution that works for me in all cases is still the by far simplest solution I found here!
Start with putting your pageset to Print on both sides:
To use this solution, you'll need 2 extra pages (I called them BackPage and Dummy).
Then you set the first page (the page that contains your data) like this:
Next you set the backpage like this:
And last the last page (Dummy) like this:
This will make sure that any blank page is set to the sales conditions. This page doesn't have a content area, only the text of the salesconditions:
Now finally very important is to tell the content to where is has to go when the page is full. It must go to the content area of the FirstPage, not the content area of the back page. If we leave the field blank, it will automatically put the items on the back page.
Now the pages will come out nicely with the sales conditions on the back, when there's only 1 page or more pages.
Just as example how you can design your sales conditions:
And the result: