For clothes of very small and very large sizes,
you may have to wait several days more.
For example,
Uniqlo sells XS and XXL to 4XL size tops and bottoms
currently only at the online store.
When such online-only items are in stock and you buy them,
it may take a few days until you can receive them at a Uniqlo real store.
Warm inner wears make you sweaty,
so it's better to prepare spare ones, I think.
Regarding Uniqlo HeatTech inner wears,
there are Ultra Warm and Extra Warm series.
Although I cannot make a simple comparison,
roughly speaking,
Ultra Warm (Chou-gokudan [ ɒg ]) wears
seem to be warmer than Extra Warm (Gokudan [ ɒg ]) wears,
and Extra Warm wears seem to be warmer
than normal HeatTech wears.
- JMA: Daily forecasts
Tokyo
https://www.jma.go.jp/bosai/forecast/#area_type=offices&area_code=1300...Hokkaido: Ishikari, Sorachi and Shiribeshi regions
https://www.jma.go.jp/bosai/forecast/#area_type=offices&area_code=0160...: Sapporo City belongs to Ishikari.
: Niseko Town belongs to Shiribeshi.