Can I read a supplier's own Shopify store?

Yes, in either of two ways:

• The store uses MostStack: it creates an invite code on its Sharing page and you enter it. You install nothing in their store and handle no keys. See I have an invite code.
• The store doesn't use MostStack: connect it through its own custom app (its client ID and secret). The link is under the code field.

You choose which of its locations count and which product statuses to read.