As you can see from the illustrations here, the stamps are full of heraldry, with flags, banners and tabards. The Wars highlighted the importance of heraldry at the time - easy identification in battle. You basically needed to know whose side you were fighting on and, equally important, who the enemy was. Hence, phrases such as 'Rallying round the flag'. Heraldry, therefore, still needed to be simple in its design as it needed to be recognisable at a distance.
The Wars of the Roses marked the end of the Medieval period in England and the ultimate triumph of the Tudor dynasty with the defeat of King Richard III at the Battle of Bosworth. A fancier, more graphic approach to heraldry, one more recongnisable to modern eyes, began with its use for propaganda as we have seen in the Blogs about the Tudors, at the same time as the Renaissance was heralded in in England. Heraldic achievements ended their life as functional symbols and took on a more symbolic role.