You can pick up a lobster roll and find somewhere to sit by the Boston Harbor Hotel or the courthouse. They're still available and as good as always.
Since they're not a restaurant, I don't know if they can shuck oysters then package them to sell. I've never seen it, but it might be worth calling them. The last time I was in a local fish market like Hook, oysters were about 1/3-1/2 the price of restaurants, but it's difficult to compare when the sources (and freshness) of the oysters may be different.