Well that's because you're paying a person and a motor vehicle's worth of fuel just to move a bottle of water. If you filled your vehicle full of water, and upgraded to a semi-trailer full of water, then its a lot less than $5 for a bottle of water.

If you launched a Starship with just a single bottle of water it'd become way more than $25,000. Similarly you could make your doordash/postmates bottle of water a lot more expensive if you moved it with a semi-trailer and a team of movers.