Hey there! This charge on account need was an important feature for merchants, especially those that want to use POS so we built an app to solve for this. We just launched an app on the app store to handle this functionality. The app gives merchants the ability to allow customers to purchase products on account and pay later within POS. It lets you have customers with an account so you can charge items to it and allow them to pay later. It also integrates with Quickbooks!
App link: https://apps.shopify.com/credit-charge-account-on-pos
Complete guide/walkthrough: https://docs.google.com/document/d/1B2s6c3vDMzQDbFOQTO6J-fHJAfXOrx2WuoUFMYWpCrw/edit