Every single bill comes with a return envelope, a window return envelope. Which means I gotta take a just a little bit of extra care to get the return slip right way round and in front of the check. Not the end of the world I suppose.
On the other hand, window envelopes ought to cost more than plain ones, if only a fraction of a cent. The only bennie to the billing company is the ability to change the address of the payment center without having to dump stocks of old pre printed envelopes. But how often does the phone company, the bank, or the electric company move a billing center?