For example, if a customer of mine wants their item to ship express, they have to send me a message letting me know, I have to respond back telling them the price to see if they want to go with that method. Then I have to either adjust the listing shipping price and reserve it for them before they buy, or they have to pay the additional through Paypal and then message me back so I know.
It would save time and buyers could choose based on their budget and needs.
This goes for any shipping method - priority, express, etc. you could offer whatever you like for your buyers.
Please consider this Etsy! I know many other sellers and buyers would love this feature!