How to unlock more items when starting a mission?

Currently, at the beginning of each mission, 4 Extra Lives and 3 Missiles are automatically added (one extra life in each slot, 3 missiles in the same slot). This takes up 5 slots in total and there is still one empty slot available. The remaing 23 slots are locked. Which item in the shop can help us unlock more slots when starting a mission? (sorry, no screenshot available so it is a bit hard to say in words)

There is an extra slot item in the shop for 212 keys, also, i think better ships starts with more slots, but i can’t check that right now.

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Yup, that’s true.