posted by Sylvie .
Do I need a comma between the words notes and so? - "Sitting at the sticky, crumb-covered cafeteria table, Luther and Ruth carefully studied their chemistry notes so that Dr. Wilson's next tricky test wouldn't confuse them."
Do I need a comma between paper and underneath? - "Annette kept a bag of corn chips hidden behind the toilet paper underneath the sink."
Do I need a comma between fish and because? - "After catching enough trout for both dinner and breakfast, we let Bobby scale and gut the fish because he doesn't mind the dirty job."
1 and 3 ... no comma needed.
I don't think a comma is needed in your 2nd sentence either, as long as the toilet paper is located underneath the sink.