I renewed mine a couple years ago online. It only takes about three clicks and you are done. If I remember correctly all you needed to do is put in your permit number then select the payment method. Mine was free since I'm 100% VA disabled. Anyone with 10% or more VA disability permits and renewals are free.
Mine is up for renewal in Q4 of this year so I may do it by mail just because there are a few things I wish to disclose to ensure no problems during the renewal process. Can't believe it's been about 4 years already!!