Heirloom Crafting Shards Question

I'm currently holding off until Valkyrie gets an heirloom, but wondered if packs could still award protentional shards on the mythic shop tab, as it shows "150/150", but also says you can still get shards as long as you still have items in the mythic shop that could be bought. I don't know how long its going to take until we get Valkyrie's heirloom, so I'm currently saving extra packs incase luck decides to exist.

Essentially, do I have to use the first set of shards first, in order for a chance to get more to become available, or are you able to hold onto more than 150 shards?