Hi Jedimistress
I think they base it on how much averagely they say you would have it. eg. They may reckon you would eat 1 toblerone a week and work it out for the year, then either send you a years supply in one go or send 1 a week or 4 a month kind of thing. I think that's how it's done anyway.
I know someone who's cat won a lifetime's supply of catfood, what they did was work out how much the average cat eats a day etc. and would supply a month at a time, she had to send a photo of her cat once a year to prove he was still alive.