Here is 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.
Everyone always invites Marc to the movies is a main clause. His jeans are so oversized is another main clause. To connect two main clauses with a comma is to create a comma splice, a major sentence error.
A period after movies and a capital H for his would take care of the problem.