would it really be cheaper
That has to be checked on case by case bases. At least when I recently ordered something via discogs every seller seems to set shipping costs at will. It only made sense to use local sellers for me her, too.
One would need to use bubble wrap envelopes, of course.
Yes, sometimes some cardboard can also work and there are also envelops with one cardboard side. Unless anyone really throws it around and breaks it anyway…