I've not had any issues with people decreasing the tip thankfully. One thing that does annoy me is if something is out of stock and I have to refund... The tip will go down if the customer has it set to a % tip instead of a flat rate. That's the only time I've gotten less than what I originally accepted the batch for.

Enhance isn't in Instacart's vocabulary unless they are talking about company profits. Preferred shoppers are already a thing, but only in some locations. My city still doesn't have it and I'm guessing yours does not either probably. If it works well, it could be a great thing. I have a friend who gets people's personal phone numbers so they can discuss when the order is about to be placed in the future because they like her so much. She's a girl though and customers aren't weirded out by her asking for their contact. I could never, I'd be getting reported no doubt. However, even with that advantage, she often still doesn't get their batches because sometimes the shopper app just decides to only show some orders to certain people for whatever reason.