At the end of each level, you will be taken to the vendor. Here, you can buy upgrades for your rock bag and medicine chest. Each affects your carrying capacity. The upgrades are as follows:

Medicine Chest
First upgrade: +5 antidote and health potion carrying capacity; total of 10 each.
Second upgrade: +5 antidote and health potion carrying capacity; total of 15 each.

Rock Bag
First upgrade: +10 rock carrying capacity; total of 20 rocks.
Second upgrade: +10 rock carrying capacity; total of 30 rocks.