Take a look at the original sentence again:
In Dr. Ribley's calculus class, Yi-Ping tapes every lecture and carefully
rewrites her notes, she is planning to sell
these guides to desperate freshmen next semester.
You chose to fix it the correct way:
In Dr. Ribley's calculus class, Yi-Ping tapes every lecture and carefully
rewrites her notes since she is planning
to sell these guides to desperate freshmen next semester.
Yi-Ping tapes every lecture and carefully rewrites her notes is a main clause. She is planning to sell these guides to desperate freshmen next semester is the second main clause. One correct way to join these two main clauses is to use a subordinate conjunction like since.