If you are doing the end-to-end and want to change your food drop from the default Ellery Creek South to Serpentine Gorge, you can do that online here: https://www.treksupport.com.au/add-extras
It costs $25 to opt for the Serpentine Food Drop option.
Unless you specifically opt and pay for the Serpentine Food drop, don't assume it is included in your food drop.