400 yen for a ten pack is more like $2.50 than $5

Typical markup in the USA is 100% from wholesaler to retail. Running brick and mortar is very expensive. So if Walgreens were selling this, the wholesale price would be $1.25. I think it reasonable to expect the Yakult Ladies are pulling in the same $1.25 per package that walgreens gets.

The key, I think, is "Most of them are self-employed". Its a gig economy idea. You have to eat. If you're walking home from the store anyway (or kids school or on the way home from work or whatever), you may as well deliver packages for $1.25 each on the way home. You're walking home anyway, you may as well make free money on the walk.