Take a look at the sentence again:
Everyone always invites Marc to the movies, his jeans are so oversized that
he can sneak in two two-liter bottles of soda,
which he straps to his thighs, saving his friends from the outrageous
drink prices at the theater.
Which he straps to his thighs is a subordinate clause that is interrupting the flow of this complete sentence: His jeans are so oversized that he can sneak in two two-liter bottles of soda, saving his friends .... An interrupter needs to have a comma both in front and behind it.
Go back to the sentence to try again.