There's no documentation for this plugin yet but the basic usage is as follows:
On start of layout, use the Add product ID action to add the product IDs you will be using, e.g. "product1,product2,product3". (This is necessary since some platforms don't support requesting a list of available product IDs.)
After that (still on start of layout), use the Request store listing action. This downloads the names and prices of the given products. (Again, this is necessary since some platforms don't support making purchases until this information has been received.)
When On store listing success triggers, the product names and prices are available, and purchases can start being made or checked with Has product.