For the "should I" question, that depends. As far as I can tell, the cash by mail option is only really useful under a few situations, such as:
You are concerned about a state level threat, using financial records on either end to determine which VPN provider you use and developing a targeted attack for that vector.
You don't trust the providers claims about account association.
You may also just prefer the smaller and more ambiguous paper trail of cash by mail. There's no harm in it, but its significance is highly dependent on your threat model.