Im sure HC will say "its a feature" like so much else :)
if you setup a addon to cost 100,- per month and when you sell it only select "billing per month" option, then its ok. But the problem is that "billing with plan" is totally misbehaving.
different problems are:
lets say you are like us. We have things like diskspace that are cheap, maybe 120,- per year (kr. so 20$ or something). This we absolutly do NOT want to have to bill and handle bookkeeping for on an monthly basis and our customers also dont want ofcause. We want things llike this to allign with billing of the plan.
so we setup the addon to cost "120" and select "bill with plan".
now its good times. We are selling!
strangely we can still select "the per month" option when selling an option to our customer. OK, so this is a feature ;) and we remember to pick "bill with plan". Now the big features of HC come into play.
WE DONT KNOW AND CONTROL WHAT THE PERIOD IS!!
so :) one customer, how is paying his website/plan per month is billed EVERY month, 120,- kr.
an other customer, how have the same plan but was lucky to pay per year, also is billed 120,- kr. per billing period - so he is paying the right amount and the other one... well, he was unlucky that we are running HC - he is paying 12 times as much. Nice doing buissness with you ;)