Stores that do not have their customers pay upfront by credit/debit card, but by invoice, are required to set up a custom payment term once goods are dispatched/shipped. This option would then be available to all of your customers, and you can manage them through Order Manager.
Visit Add a custom payment method in Shopify for detailed instructions.
Bold Order Manager is no longer available to install on the Shopify App Store. If you already have Bold Order Manager installed, you will be able to continue using it as it is grandfathered on your store.