Configure Magento settings
Go to Stores > Configuration > Sales > Order Numbers to configure your chosen settings in Magento.
Invoice/Shipment/Creditmemo Prefix (v1.0.0+)
Choose a prefix for your invoices (e.g. INV-), shipment (e.g. SHIP-) and credit memo (e.g. CREDIT-) numbers.
How To Use The Extension
Once installed, all new invoices, shipments and credit memos will be automatically assigned the same number as the related order. For example, for order #1000, the associated invoice would be #1000, the shipment #1000, and the credit memo also #1000.
In the event that there are multiple invoices, shipments and/or credit memos associated with the same order, duplicate separation will happen automatically. For example, the first invoice would be #1000, the second #1000-1, the third #1000-2, and so on.