Originally, Liu Zhizhong intended to act alone, not wanting to bring anyone along.
However, Grandpa Lou has difficulty with his legs and feet, and nearby villages often have patients in need of him, so he can't really leave.
Additionally, there's been a recent shortage of medicine, and his clinic needs replenishment, so he asked Ruan Yu and Liu Zhizhong to go to town together to procure some Western medicine.
As for herbal medicine, there are villagers in the rainforest who gather herbs, which can be entirely purchased.
Liu Zhizhong initially thought he could just help with the procurement, but then considered that he didn't have any money on him; what if Grandpa Lou worried he'd run off with the funds?
After all, Grandpa Lou handed Ruan Yu 6000 M Coins to buy medicine. Such an amount indeed counts as Big Money.
M Coins used in Vallebang have a fairly good exchange rate compared to domestic currency; they can usually still go for 1 to 1!
