The tables with d% entries on them on this page are used by the GM for randomly generating magic armor, like if the GM's filling a random treasure hoard in a dungeon or stocking a shop; they give the GM an idea of what magic armor special abilities are approximately equivalent. As a player, though, you only need to worry about Table: Armor and Shield Pricing by Bonus. Those other tables don't change that table.
Further, when a magic armor, shield, or weapon special ability has a flat price—a price that's measured in gp rather than magical pluses—that magic special ability can be added to that item by paying that price. No chart need be consulted.
(In both cases, there may still be the matters of both finding a character who can add the magic special abilities to the item and finding the time to have those magic special abilities added, but those're matters for the campaign not this answer. If you're making a new character—NPC or PC—who's entering play with above-level-1 gear, you can usually ignore that.)
Thus adding the magic armor special ability improved slick to any already magic +1 or better armor—because armor (like shields and weapons) must already be at least that magical before they can be made even more magical—means paying 15,000 gp, regardless of the present total pluses of the magic armor.
Sort of similarly, adding the magic armor special ability balanced to any already magic +1 or better armor means just paying the difference between the magic armor's current pluses and the final pluses it'll have when the new ability is added. In this case, that's the difference between +5 magic armor and +6 armor, so that price is 11,000 gp.
In sum, adding both the magic armor special ability balanced and improved slick to +5 magic armor means paying for both—a total of 26,000 gp—and the armor will end up being +5 balanced improved slick armor. If the owner wanted another magic armor special ability added to the armor, and that magic armor special ability is measured in pluses, the owner'd pay the difference between the new pluses and the current armor's total bonus of +6 (i.e. the +5 enhancement bonus and the virtual +1 from the magic armor special ability balanced).