I remember the prices seeming comparatively much higher in Alaska/Hawaii in the past. Fast food prices have gone up so much everywhere that shipping to these places likely represents a lower percentage of the total cost now.
>identical other than supply shipping costs
And taxes, labor rules, insurance rates, operating hour limits, ...
Don't forget rent and energy. They like to gripe about the cost of labour and labour regulations, but the biggest share of operating expenses is rent, then energy.