Zendesk is a customer support tool. It's got 2 main features: ticketing and knowledge base. Ticketing: Zendesk gathers customer queries in one place where you can manage and respond to customer questions efficiently. Knowledge base allows you to create an FAQ section to guide your customers.
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.