Seems a bit if a rip off to me.
£20 arrangement fee so for me given the price of the two season tickets I buy it represents just over a 5% premium on that.
Also if you book online the calculator adds a £2.50 per ticket "card fee" to the price despite the fact you are not paying by card.
It even does this for the car park ticket if you buy one of those as well.
So to put two season tickets and a car park ticket on the plan would have the additional cost of £27.50.
It is NOT a 0% payment plan and the club should be shot for publicising it as such.