If you're looking to streamline the buying process for your customers on Shopify, Shopify Plus, or BigCommerce one great way to do it is by using a popup or fly-out with a button that enables shoppers to add a pre-determined product to their cart.
First, insert a button element from the elements menu. Then click to edit the link.
For Shopify stores:
When you insert the link, you just need to use a special permalink that includes your store URL, cart, the product ID or variant id, and the quantity.
For example - http://my-store.com/cart/add?id=123456789&quantity=1
Need help finding your Shopify product or variant id? Click here
For BigCommerce stores:
To automatically add a product or variant to a shopper's cart and take them directly to the cart page, include cart.php?action=add&sku=INSERT-SKU-HERE after your store's domain.