The $2/mo is for access to additional storage. If you don't pay your $2, you lose access to that storage. People could manipulate the system far too easily if they were allowed to pay $2 for a single month, load up that retainer with items then slowly withdraw those items over time as desired.

You need to start withdrawing the items before you let the retainer subscription expire if you're going to be using those items. Or keep paying the extra $2/mo. Not like that's a huge amount of money. All you have to do is skip buying a drink when you go out to eat once a month and you've paid for it.

Also, this isn't designed as a temporary inventory system. The game isn't the one deciding to give you temporary access for special purposes. You're the one choosing to make the purchase.