Can we defer using cards after seeing them in a lunchbox?

Solution 1:

They are used immediately, but not wasted

Cards from an opened lunchbox are immediately applied, however the game does its best to not waste your hard earned lunchbox draws by adding what you get on top of your existing things, even if it puts you over capacity. The excess stays as a "bonus", it doesn't get discarded.

Food, water, electricity, and caps go directly to your current vault supplies. However, the extra amount from the lunchbox cards is not lost, it will just put you temporarily over capacity on the resource. For example: if you were at 199/200 food and pulled a 50 food card, you would have 249/200 food

Outfits, weapons, stimpacks, and radaways go directly to storage and may also put you over capacity. Having more stimpacks and radaways doesn't seem to have any downside; they are usable on dwellers and can be sent with explorers just like normal ones.

However, you will not have access to new outfits or weapons if you are over capacity in storage. The excess items will not be deleted, but they will be darkened and have a dashed outline until your number of gear is less than or equal to your storage. To free up some storage capacity, you can build or upgrade more Storage rooms to add capacity, you can equip dwellers with items, or you can sell unwanted items for caps. Aside from not being able to access that sweet new loot from the lunchbox, being at or over capacity will prevent any returned explorers with items from entering the vault until you deal with the lack of storage capacity. Note that you can sell items directly from the explorer's inventory by tapping on the item, however this only works when they are waiting in line at the vault entrance. Once the explorer's items plus your items in storage are within the capacity, you will be allowed to bring the explorer back inside.

New dwellers (and Mr. Handys) go directly to the line waiting to enter the vault. This line has a max capacity of 10 waiting dwellers, however the game will prevent you from opening more lunchboxes if the line has 10 people/robots in it. To free up line space, let them into your vault. You can send them out to die exploring the wasteland if they don't meet your vault's high standards, but they'll be out of the line so you can open more lunchboxes in your quest for that glorious Guided Fat Man.

Source for all of this is opening way too many lunchboxes.