This was executed when connected to a good high-speed Internet
connection:

  $ aptitude upgrade -D

That fetched all pkgs without installing them. Then a week later when
on a capped Internet connection, aptitude was executed with no
arguments to launch the TUI. This was done because the CLI gives no
way to selectively exclude a package from the upgrade for an operation
for just one execution. The TUI makes it possible to exclude a package
on a one-off basis. These steps were followed:

  $ aptitude
  Entered “+” on the “Upgradeable” line
  Expanded the tree and highlighted a pkg to exclude from the upgrade
  Entered “:” to unmark a pkg
  Entered “u” on the “Upgradeable” line

It proceeded to download /everything/. Internet credit was getting
sucked dry. Why?  I had to quickly kill the operation. Then I ran this:

  $ aptitude upgrade --simulate

The output printed:

  “Need to get 1,979 kB/567 MB of archives,”

That means only a small amount needs to fetched. Just two megs, likely
a new prospective upgrade was detected the 1 week lag between the
download. But the TUI was re-downloading all 567mb. The aptitude TUI
and CLI should be storing the data in the same location.