The way you have it phrased now comes off horribly rude. If you chose to send the letter - don't mention past gifts. That comes off as being ungreatful for what they've already bought. I kind of like the pp - send a small list of ideas with a note that says something to the effect 'here some present ideas XXX would really enjoy for Christmas if you need some ideas. If you've already done your shopping, just ignore this list. Merry Christmas!'