Increase quest items inventory space?

[Ze Mwape Evaristo]

I know you can delete the reward items (the ones that increase party stats or sell price in an area) and take them again when needed.
It's still annoying because you have 6 out of 20 spaces occupied by flowers, a few occupied by notes of commendation, maybe a letter for the mailman and a few potions.
So there are only 10 spaces left for buffs, and in the late game I want to be strong, I don't want to find myself going around removing and re-equpping buffs.