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.
Refersion is an affiliate tracking platform that manages your influencers and affiliates. It integrates with major eCommerce platforms and provides you with a place to bring on affiliates, create different offers, calculate commissions, and track how much an affiliate needs to be paid.
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.