On delivery apps it should be called a "bid" and I think honestly it would be more ethical than the shady nonsense they do now if they went all in on that.
i like the idea of bids, but only if its coupled with the ability of workers to just outright reject the offer.
the problem is that there's still a guarantee of service for the user regardless of what you bid. if i want my burritos delivered at 2AM and am only willing to bid $1 on delivery labor, i shouldn't expect that my burritos will be delivered.
for tips, the model is "the work will be done and i'll pay extra if it's good". but with bids its "how much do i need to pay to get this work started"
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.