Or gambling? It's the lootbox of food...
With usps you are paying more for a guaranteed service. There are standards they'll hold that delivery to. You are paying for something specific and real.
A higher pre-tip on the other hand is paying for a hope and a prayer that you get what you paid for on time and in good working order. It's not the same thing at all.
If you paid a fee for priority Uber that let you jump the queue it'd be closer to not a bribe.
Do you feel the same way about how offering a better price on craigslist gets you counterparties faster? eg. if the going price for a phone is $300, and you list at $250, you're going to get people messaging you sooner because you essentially "jumped the queue" for everyone who's looking to buy a phone.
With Uber you are making a one-sided negotiation in which the other party doesn't know what terms you'd like. You can pay more to potentially get better terms, but you don't know how much more to pay or how much better it will make those terms.