Inter-island flights in Hawaii: Lower 48 ticketing model or bus model?

Do the inter-island flights in Hawaii use
- a ticketing model like the lower 48 states (purchasing far in advance is massively cheaper, high fees to change tickets)
- or a "bus model" like the inter-island flights in Trinidad and Tobago (show up, buy a ticket, get on the next plane)?
I'm planning an extended trip to the islands, and I'd like to maximize my flexibility by not buying inter-island flights until I have exhausted the charms of the island I'm currently on. But, not if it costs me a fortune to do so.
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Mokulele Airlines | Hawaii's Inter-Island Airline
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Images: Aidan Roof, Cleyton Ewerton, Ronailson Santos, Cleyton Ewerton