The Checkout process starts when you tap on any product listed in the shopping post. You will then be forwarded to a selection page with various options for said product, with the “Checkout” button located at the bottom the page.
— Instagram (@instagram) March 19, 2019 Before making the order, it is a requirement that you provide your delivery information and choose a payment method. For now, the feature supports only credit cards and PayPal. Users can also save their payment information for future transactions as well. Once the order is completed, users can track their shipment and delivery directly through the app itself. Instagram will also send a notification the minute the retailer ships their order.
For the moment, Instagram Checkout is currently a closed beta, and only available within the US. At the of writing, Instagram has not released any information regarding the availability of the feature for other parts of the world. (Source: Instagram, Engadget)
