Another option would be to mail a physical letter to their billing department stating that you are cancelling your subscription 30 days from now and any subsequent charges to the credit card will be disputed.
I live in the middle of nowhere (population around 200) and i can buy stamps with a 10 minute walk; envelopes, labels, boxes, etc as well. And if that post office is closed, there's another one 10 minutes up the road, and if that one is out, i can drive a triangle to get to another one in about 10 minutes.
Larger cities may require more time to get to a post office, but there's probably 5 places between you and the post office that also sell stamps and envelopes.