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The whys & wherefores of how this app works is irrelevant, it is a maths question. I never said the driver had a degree, is actually his wife apparently, who agrees with his maths. That's why I mentioned it.
If it helps, people use this app to have someone do their shopping. The driver does the shopping, pays for it & is then reimbursed that cost & paid a fee for their time (& mileage) on top.
This driver is trying to say that if the customer is in cahoots with the driver, they'll end up getting the goods much cheaper. I say they'd get them cheaper by the amount of the fee (if the driver does it for nothing !). He says it'd be cheaper by the actual cost of the goods plus the fee, ie. £70 (Prudie, the £70 reimbursed already includes the £20 fee, you've added it again, hence £80).