I don't agree with that. When I signed my contract with Verizon, they offered online payments via credit card as a free-of-charge option for payment. By charging for that, they are materially changing the nature of our agreement, regardless of whether it was in the contract.
Take it to the extreme: Since method of payment isn't stipulated at all in the agreement, they could just as easily say "Starting 1/15/2012, any payments not made in person at our payment processing facility in Kodiak, Alaska will be subject to a $100 convenience fee." Would that be ok? At what point does it become not ok?
IANAL, but I would expect the uniform commercial code to cover what payment methods are allowed if not stipulated in a contract. For example, in § 2-511 of the UCC (http://www.law.cornell.edu/ucc/2/article2.htm#s2-511) "Tender of payment is sufficient when made by any means or in any manner current in the ordinary course of business unless the seller demands payment in legal tender and gives any extension of time reasonably necessary to procure it."
I'm not sure if leaving out such a demand in the contract bars them from making one in the future, or if a court would recognize such a right when it is unreasonably burdensome on the part of the consumer, however - or if they would allow the payer to offset their costs incurred in delivering the legal tender to the payee. That said, if anyone knows of case history regarding this, it would be very interesting to read :)
Take it to the extreme: Since method of payment isn't stipulated at all in the agreement, they could just as easily say "Starting 1/15/2012, any payments not made in person at our payment processing facility in Kodiak, Alaska will be subject to a $100 convenience fee." Would that be ok? At what point does it become not ok?