Shopify is a commerce platform that allows anyone to set up an online store and sell their products. It's the go-to eCommerce platform for young direct-to-consumer brands. It's ease of use and setup is what's most attractive to merchants, but also the incredibly vast Shopify app store.
Postscript is a text marketing solution. Collect subscribers at checkout, on landing pages, using mobile pop-ups and QR codes. Then - Automatically send text messages based on purchase data. Follow up on purchases, abandoned carts, and more by activating default templates or creating new ones.
Klaviyo is the best in class email marketing solution for eCommerce. It's powerful because it connects to your store, and you can create awesome customer segments based on past purchasing behaviors, customer preferences, and even future predictions of spending.