Buying snacks from departure area in Naha Airport

hi! visiting the sunny island soon 😀

I don't have checked baggage and is only allowed 7kg carryon with Jetstar. I'm wondering if I buy souvenirs and snacks from the duty free in the airport instead, would it be excluded from the 7kg combined weight limit? Otherwise I might have to purchase add on baggage as I read Jetstar is quite strict with baggage.

And also if I can enter the domestic departure duty free area if I'm flying international?

Cheers!