Sizes, weights and other variants
When one product is really several, and how the price follows.
Where: Dashboard, Catalog, a product
A variant is the same product in a different size, weight or pack. Each carries its own price and its own stock, and the shopper chooses one before adding to their bag.
If you set a low-stock level on a product, each size shows how few of it are left once it drops to that level, and a size that has sold out is struck through but still listed, so a shopper can see you stock it.
Once a shopper scrolls past the buy button on a product page, a bar at the bottom of the screen keeps the product within reach. After they pick an option it names that option and its price, and its button adds that option to the bag. If the option they picked is sold out, the bar says so and does not add it. Until they pick one, it takes them back to the options.
On a product page the bar is there whenever the buy button is off the screen, which on a phone can be from the moment the page opens. On a home page that features one product, as Kaksha's does and Lipi's featured course does, the bar waits until the shopper has scrolled past that product's own buy button, and goes away again if they scroll back above it.