Split the parcel with flatmates. Purchasing agents charge a flat handling fee of roughly ¥8-12 per parcel on top of a regional base charge of ¥18-20, and the per-kilo rate is identical whether the box is yours alone or shared. Three people consolidating into one 6kg parcel to Western Europe pay about ¥270 of freight between them, against roughly ¥400 shipping separately. It also means one customs event and one delivery to chase rather than three.
Order before term starts, not during freshers' week. Realistic timing is 2-5 days for sellers to reach the agent's warehouse, a day or two for quality checks, then 7-14 days of line-haul to Western Europe or 8-16 to North America, plus customs. Three to five weeks door to door is normal. Most agents give 60-90 days of free storage, so you can order items as you find them over several weeks and ship everything once, which is both cheaper and far less stressful than a panic order in September.
Spend more on the two things you touch every day: the desk lamp and the chair cushion. Everything else in a dorm is genuinely fine at the cheap end, and the honest caveat is that some of it will not survive the year: expect a collapsible box to sag, a cheap extension lead to be worth replacing, and at least one item to be the wrong size because you measured the desk from memory. Budget an extra 10% for replacements and treat it as the cost of furnishing a room for under ¥1,300.