thanks for picking up my issue. Please check again. I need to sum up elements <= ZIP_all, which results in a collecion of items. If it were =ZIP_all it would surely work like that.
I now did it with two dimensions ZIP_all and ZIP_codes and used as a formula:
sum(if(ZIP_ALL <= ZIP_code), sales_amount)
And then pivoted it with ZIP_all. That seems to work. But I feel still unsure if it really does and if this is the best solution. I think my problem is similar to this one: http://community.qlik.com/forums/p/32278/124939.aspx
As John Witherspoon says in that thread, set analysis is performed once for the whole model, not by chart dimension, so you can rule that out as the solution to your problem.
That leaves you with the sum-if approach. Without seeing your model, I can't be sure that what you outlined is correct, but if not, you seem to me to be on the right track.
If you would like to post your QVW file, I would be happy to have a look at it and possibly make a better suggestion.