Can the letter as an object tell us anything? Can its physical appearance tell us anything?
A quill would have been used but paper, rather than parchment, is the material on which the letters are written. As early as the thirteenth century, there were established paper mills in Spain and Italy, and in France by about 1340, Germany by 1390. However, in England paper mills did not appear until late in the fifteenth century. Before their establishment in England, from the fourteenth century onwards, paper was imported from Europe.
The Pastons typify the move that was common in the fifteenth century to a change in writing material to cheaper material. By this century, parchment was not used for impermanent records because, quite simply, it was just too expensive. Only records designated to be permanent records were being written on parchment. The fact that paper is used reinforces the notion that the Paston letters were never intended for posterity.
The irony, though, is that medieval paper lasts. It is much more durable than the paper we use today. In the Middle Ages, paper was made from linen rags. Linen is much stronger than modern wood-pulp paper. As a result, many medieval manuscripts written on paper have survived to today. Cheap small sermon books, text books, popular tracts and so on, made for clerics and students, were more often written on paper than on parchment by the fifteenth century.