Here is the item again:
His arms full of expensive science fiction
paperbacks,
(A) Cary
remembered (B)
that next semester's tuition was due in two
weeks. (C)
Making a trip to the library instead of the bookstore a better financial
decision.
You correctly realized that a comma was required after weeks. Making a trip to the library ... is a participle phrase fragment, and when you connect such a phrase to the end of a main clause, you will often need a comma.
To fix this problem, you chose to do this:
His arms full of expensive science fiction paperbacks,
Cary remembered that next semester's tuition was due in two
weeks,
making a trip to the library instead of the bookstore a better financial
decision.
You have correctly employed Punctuation Rule 3:
Main Clause + , + Participle Phrase.
We will hum that tune all day!