While I agree that marginal probability is important, I think it is a fallacy to assume all of those other costs don't have some variable component as well. There will always be shopper worker churn that will grow as the business grows, additional (albeit low) costs in software to scale and add new regions, etc.

In a business with sizable margins, it may be OK to discount some of these other things, but in a business with teeny tiny razor thin margins like grocery delivery, one should have a very healthy skepticism about hand-waving away real costs.