The Sunset Bar Shuttle goes to Pushcart and it only takes about an hour and a half to two hours total. The driver sits out front and waits for you.
Not sure about Margaritaville trip, but it's probably close to the same.
I've heard from several folks at the resort to skip the shopping trip, but never done it personally. We have walked across the street to the craft market before, which was cool. Just be ready to haggle and say no thank you a lot.
My wife and I enjoy shopping the local crafts. I would rather buy from them instead of giving my money to a "retail" store. That being said, the pressure is so high at the market, it takes a lot of the fun out of it.