300 Yen no Otsukiai

Based on the adult manga by Shouji Ayumu
Even though she is 18 years old, Manami-chan loves snack foods, and behaves like a child. Our protagonist meets Manami at the supermarket and upon noticing her bad mood, buys her snacks out of pity. However, those snacks are attached to one condition: that Manami-chan has sex with him. Snacks, sex, snacks, sex. This pattern repeats until one day, something happens...
(Source: Nekopoi and Has Been Translated)