First off, Postmates does charge menu pricing. Postmate Couriers are supposed to give you an itemized receipt, just as if you had got it yourself from the restaurant. The app does indicate that prices may vary, notably on fast food places. So, you are forewarned, but check the receipt. You are simply charged the delivery fee, which varies based on distance and complexity, as well as time of day. There also is the service fee. Delivery charges and such are the norm.
FWIW, Postmates really is designed in a way that you are just having a surrogate go in your place. Your basically hiring an assistant to do a chore of picking something up for you. For the most part, restaurants like it, because it does increase sales. Many restaurants are on all the platforms -- Postmates, DoorDash and GrubHub. There were some issues over DoorDash and those restaurants did get removed.
All in all, this is the wave of the future. On-demand services are here to stay. Its all part of our society and cultural norm now. Postmates is a great app. Service is great and will only get better.