DSF

Wonder if it would be worth vacuum sealing the bags lol - they should still pop up as normal when opened shouldn't they ?

I really think that the crux of the matter here is that if the item is a certain size then it will be treated as a large letter and then a packet. The Royal Mail says a letter is up to 240 x 165mm with maximum thickness of 5mm and weight of 100g, Large Letter is up to 353 x 250mm with maximum thickness of 25mm and weight of 750g. Above this they are packets (and then finally a parcel).