Sup guys, name's Jay and i'm on Oahu. Many times if u need larger items shipped in cheaper theres a few ways to do it.
1) For gallon sized or larger boxed items that Pakshak doesn't carry, you can contact Ranney or Ron and maybe they can order it for you for their next pallet. I've done this with them before and its mutually beneficial.
2) Order online with businesses that can ship "Parcel Post" from the USPS. This is the cheapest way to ship to the islands, but takes longer. I think the longest I waited was 2-3 weeks. I order 5 gal containers shipped from the mainland for about $55 each multiple times a year. The key is, to order it well in advance that the shipping time doesn't matter. with UPS or Fedex you're paying way too much.