FYI - There are only certain flights you can take with zipair if you are not allowed entry in to Japan that allow you to transit without going through immigration (only posting the LAX-based flights):
Regarding Transfers and Same-day Connection Services | ZIPAIR Official Website
For customers transferring via ZIPAIR to a different airline, all travelers must go through immigration upon arrival, pick up your checked baggage, and go through Customs inspection. You may be required to provide mandatory documents for entering the country of transit. If you cannot provide the documents, boarding may be refused. Please prepare all travel documents in advance.
Customers with same-day connections on select flights operated by ZIPAIR are not required to go through immigration and can transit through Narita airport. Applicable connections are listed below:
ZG23 (Los Angeles-Narita) → ZG51 (Narita-Bangkok)
ZG23 (Los Angeles-Narita) → ZG53 (Narita-Singapore)
Aside from those specific transfers, it's understood that you will be forced to go through immigration even if you're carryon only (I haven't seen any reports that say otherwise), and you will only get refunded a tiny amount (taxes - like $65) if you have to cancel. So buyer beware.
That said, this flight at times will be as low as $400ish each direction as they are a LCC. You can also find ANA and JL with $500 flights each direction to/from LAX and NRT/HND.