By Goblet of Fire, we don't really know why the Dursleys insist on giving Harry Christmas presents – they might as well save themselves the effort. In Book 4, they hit "an all-time low" by giving Harry "a single tissue" (23.49). The simple fact that the Dursleys bother to send Harry a single tissue, rather than just ignoring him entirely over Christmas, suggests that their gift-giving is meaningful. We just don't know what it symbolizes yet. We'll just have to keep reading the series to find out!