U21/NT players only get extra merch IF they are listed in the lineup for the game day. In short - being in the team means nothing. They have to be suited up (they get the bonus even if they don't play any minutes, as long as they just sit on the bench.
As for the amount - this info is just purely speculation and estimation, it is not necessarily accurate, but home grown NT players should get 15% if they are playing for a WR #1 NT. Every spot lower would get a 0.08% decrease. You can see the discussion here:
(317998.1).