I always used OGC. Charged me $25 for veteran/LE discount ... regular price $40.
Depending on the seller, they may already have your selected FFL's license on file. That makes it convenient.
If you use OGC, you can download a signed copy of their license to forward to the seller if they don't already have it.
Some Hawaii FFLs have a policy to only receive firearm shipments from other FFLs. Too many inexperienced private sellers cause problems when shipping guns. Make sure whichever local FFL you pick accepts firearms from non-FFLs if that's applicable to your purchase.
As I posted on other threads, you may want to factor in more than just FFL transfer fees. As mentioned, look into their storage fee policies. OGC implemented a fee for storage after 90 days, but they've waived that fee during the pandemic as long as you are actively working to get an appointment as soon as possible.
Another factor is location. Don't force yourself to have to leave work, drive downtown to pickup your permit, then drive across the island to pickup the gun, then back to work. You might also want to stop at home between the shop and work if keeping the gun in your car at work is prohibited. Even if it's allowed, there's the potential risk of theft.
Another thing to consider is to whom you normally give most of your shooting-related business. It makes sense to spend your money where the service, variety, and prices keep you coming back. Taking that FFL transfer fee away from them after you just bought a gun from another source takes all that revenue away from them. I'd try to support them as much as I can even if they couldn't compete on the firearm purchase itself. If they can't stay in business, it affects you, too.
Those are just a few things to consider. There are threads on here that lay out the details of making online purchases from Gun Broker auctions, dealers, and just private sellers advertising on the Internet. If you have specific questions, I'm sure someone has the experience to help.