Fallout – Under what conditions do stores replenish

A 300 cap store tends to see 25-30 caps per day in income with a moderate Charisma score and no investment in anything that may raise the amount of caps earned. That puts the ROI at about ten in-game days or so for the basic general store. However, bear in mind that stores also increase the happiness of settlers which has a different kind of game value altogether.

Can't prove a negative, but I've never seen any shipments from the vendors that I build. I've stocked several with tier 4 general store vendors and they still don't have any shipments of materials.

As for Bunker Hill, it becomes a possible settlement point after a certain quest:

The Battle for Bunker Hill

And there are shopkeepers there that sells some shipment items. It seems to be the only possible permanent way of purchasing shipments while on a settlement.

Update:

There are a few other possible settlements with a npc vendor that sells some shipments:

Abernathy farm and covenant. However for covenant, you need to:

Side with Dr Chambers in order to not have to kill the town.