We've had an 80% failure rate of delivery to Hawaii when using the Post Office, so we've had to temporarily suspend USPS shipping completely. The problem is these packages usually go out with a tracking number that isn't updated despite what USPS says they will update on the tracking number so it only reads as "The package has been picked up and is on it's way for delivery." for weeks without status updates on where it actually is.
Then what typically happens is we get a label about 6 months later saying "oh yeah, we found this random label in our facility in Long Beach (or where ever)" and a note saying that an "investigation is taking place."
We do at the moment offer UPS Ground to Hawaii, and we may need to change some variables on the website, but we worked hard to get some pretty substantial cuts to the rate with our rep, which evidently was a big deal for them to do since it's almost treated like a foreign company from the 48 states.