I know similar questions have been asked before but this is slightly unusual. One of my children attends a sports club also attended by adults. One couple bought them 2 Christmas gifts "because we thought of you when we saw them". The gifts are extremely well chosen, I wish their relatives took such care. There is no tradition of giving each other gifts other than a "secret santa" type where you buy a present suitable for anyone so it was simply an act of kindness.
I feel that this merits either a thank you letter or at least a card rather than simple thnaks, especially from a not very verbal teenager.