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.
Klarna gives your customers flexibility on payment when purchasing from your store. Customers can split their purchase into manageable installments or pay for your order up to 30 days later. The merchant cost for each purchase is up to 0.30 USD + 3.29% per transaction.
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.