Portal slide up is working fine but problem is when printing, given that I have a long portal, it prints blank pages even though they're empty
Did you select the "resize enclosing part" setting? that is supposed to remove the empty space produced when a sliding object shrinks.
Now, I tried using a list view, but for the life of me I can't figure out how to maintain my format wherein the top section will contain the latest bill while the bottom section of the layout will contain all other unpaid bills
This sounds like something that could be done with a) the correct find or Go to related records to bring up a found set of the correct records, b) sorting the records so that they group into the two groups you describe with c) sub summary layout parts serving as "sub headers" and "sub footers" above and below each group of records.
But that is only possible with certain data models, so to explore that option would require a much more detailed description of the tables and relationships involved than you have posted here.