Average revenue per user (ARPU) tells you the average of the actual to-date spend of all users in a particular segment (could be your entire customers base). Note that ARPU is a "backward"-looking metric, as opposed to a "forward"-looking metric like customer lifetime value, which includes expected future spend.
In Segments, we use net revenue and all customers who have purchased any non-fully-refunded order to calculate ARPU.
ARPU = Net revenue / Customers
In Shopify, ARPU can be derived from subtotal_price and orders_count.