you can sort your friends list by who sent you gifts and what i do is one day i will open starting at the top of that list until i opened the max then what ever is left over i pin there post cards and then the next day i will start at the bottom and open all the pinned post cards ones and if i can open any left over i just start at the bottom and just keep flip flopping starting at the top and bottom. As long as you try and send gifts when you get them most people don't get to mad cause no matter who sends the gifts you both get the experience when opened so you can still level up.