Originally Posted by
turnips
AirAsia Japan hasn't existed for some time so no chance of getting caught there.
Not sure about Peach, Jetstar do weigh at check-in and at the gate and will charge you about 5,500 JPY if over... but they also let you upgrade to 14kg carry-on for a fee of about 2,000 JPY by memory.
Peach will also weigh at check in and strictly adhere to the baggage limit. Peach's carry on limit is stated as "Up to 7 kg for 2 items" and I don't think you can upgrade. Checked baggage is however quite reasonably priced for domestic flights.