I highly recommend the article Maximize Your eCommerce Profits with Cross-Selling* from the Official WooCommerce Blog*. It was written by Stephen Roe on August 9, 2018.
An interesting set of questions recently came up: How best to take phone orders, change new orders just in, or append add-on items or orders to a customer’s account using stored payment. This is akin to performing a post purchase up-sell or cross-sell administratively.
I highly recommend the article Tips to Optimize Post-Purchase Experience for WooCommerce from the Official WooCommerce Blog by Cody Landefeld on March 8, 2018.