We buy potting soil in La Caterina outside Masaya. A huge white bag sells for not much. The plant sellers have them stacked by the road. You can cut a deal for several bags. I don’t think I have ever seen potting at PriceSmart.
I am not sure PriceSmart is the best place to buy in bulk for some of what you want. I would ask around for a vendor at one of the mercardos who might give you a better deal on the rice and beans. The red beans are currently very expensive as there is a shortage. The government is encouraging people to buy black beans, but they are not having much luck. Ask a Nica who works with your group about buying in the mercados, or, make friends with a taxi driver.