Your need to choose and deliver a wedding gift for your boss's sister by Sunday at 11:00 am. You haven't even picked it out and the weekend is here. You ...
Wait until Sunday morning. If everything goes well, you should be able to pop by some store to get it and get it to the wedding by 11:00 or so.
Purchase the present online on Saturday morning, and ask that it be giftwrapped for pickup by noon. You bring giftwrap with you, just in case, pick up the present, and deliver it to the wedding location so you can relax on Sunday.
Purchase the present online so you can pick it up and deliver it Sunday morning. They can probably giftwrap it there. If not, you can buy gift paper and wrap it yourself on the way to the wedding on Sunday.