Is this sentence correct?

Help me with grading:

Marci's old car cost too much to repair so she bought a new car, but her new car is smaller than her old car.

I believe a second comma is needed after repair so it's read as "...repair, so she bought a new car, but..."

Is the sentence correct if a comma is put after repair, and incorrect if there is no comma after repair? Or is neither correct and it should be written another way?

Thanks in advance!