Flex Washers are used with the rivets to attach them securely to the boots. Never use rivets on their own if washers are supposed to be included.
Single washers are used for the pastern strap under the ball rivet. These are sold in a pack of two (enough for a pair of boots).
Double washers are used for the shell of the boot under the ball rivets. These are sold in a pack of four (enough for a pair of boots).