They use boxes because:
Posties who’s bags are full, left the jiffybags behind at the depot instead of delivering them.
In town, all boxes are delivered by a separate van so issue above is removed. (the fact that you still get a “you weren’t in” card doesn’t seem to factor)
Boxes fit better into the lorries collecting from them, saving at least 1 lorry a day, so environmental reasons (bet they got a cheaper rate too).
Automatic packing saves man power.
Cardboard is recyclable, jiffy bags aren’t as easy to, normally end up in waste.
Sure there were some others that I read in some article, basically its just cheaper for them.