If the customer is ordering products you sell , use an estimate. If you want you can customize an estimate using a custom field where you can type in the customer PO number
If it is a blanket PO for x-amount, use an estimate and a service item, but enter the amount as the qty and the rate as $1. That will allow you to use part of the $ amount per invoice
Customer's Purchase Order feature is also commonly known as 'Sales Order' for your company which QuickBooks Online (QBO) does not have yet. They have 'Estimate' feature (which is a different feature) is available in QBO. With Estimate form, you can create Progress Invoicing (help to create multiple partial invoices from a single estimate) and also be able to convert Estimate to Vendor Purchase Order if needed.
Customer's Purchase Order feature is also commonly known as 'Sales Order' for your company which QuickBooks Online (QBO) does not have yet. They have 'Estimate' feature (which is a different feature) is available in QBO. With Estimate form, you can create Progress Invoicing (help to create multiple partial invoices from a single estimate) and also be able to convert Estimate to Vendor Purchase Order if needed.
One of my businesses deals with customer purchase orders routinely. In QB a sales order is just a record of an order being placed. A customer PO on the other hand is an authorization to purchase and warrants that the funds have been reserved. Get a customer PO and do not enter the customer PO number on your invoice and it gets sent back or trashed, depending on how the A/P clerk receiving it feels at the time.