How do I enter a customer's Purchase Order which will allow me to raise several invoices over a time period against it. Keeping a running total of what's been previously

Hi donna-hynes, thanks for joining the Community

The purchase order feature in QuickBooks Online is for suppliers only.

Instead, you can create an estimate for your customer, and then convert this (the total amount, or a specified amount or % of each item line) into an invoice. The estimate will remain open until the full amount has been applied. You can also run reports, including the Estimates by Customer report, for an easy overview of the status and balances.