For example: in this current hunt, you can buy up to 25 boxes in the store and open one for 149EC. If I open one and have left over shards from the box, are they useless after the hunt? Is there an option to get the avatar after the hunt has passed?
You can see in store when those such shards will expire (i.e. when the avatars leave store), which is 18 days and 5 hours for the current ones. As such, previous shards can no longer be used to buy avis, unless there is an activator in the store, which is the case for zodiac and birthstone shards (those will not "expire").