If the apps were actually what they claim to be (a marketplace to connect hirers and workers) that's exactly what you'd have - the cost of the food + flat fee for the platform + your bid for the work. The platform could even suggest a bid, but you could choose to bid higher or lower depending on what you think would go through.
But that's not what they are; they're shady "avoid employment laws" companies.
I often see the claim that it would be above board/legit/not a circumvention of employment laws if they ran a bidding system like that. But ... that's exactly how Sidecar worked (Uber competitor, shut down Dec 2014), and I never saw anyone distinguish them from Uber in their criticisms (until people forgot that model existed).