In Freeagent I could set custom invoicing number sequences -per customer- (eg. ABC001, 002..for Co.ABC, RAY001 for Co. RAY etc). How to do this in QBO _per customer_?

You're correct, @IUERTP123. Setting up custom transaction numbers will affect all customers.

I know how beneficial it is to set a custom transaction number for each customer. Since this feature isn't available yet in QuickBooks Online, I recommend submitting your product preference to our product developers.

This way, we can boost your QuickBooks experience. Our engineers can review your suggestion and might add this functionality in future updates. Here's how you can submit a product suggestion:

1. Click the Gear icon on the top right.
2. Select Feedback under Profile.
3. Enter your product suggestion.
4. Hit Next to submit your request.

You can reference this article for other details like turning off custom transaction numbers: Add custom transaction numbers to sales forms in QuickBooks Online.

In case you want to get customers' outstanding balances who are falling behind their payments, you can run an accounts receivable ageing report to check it.

Let me know if you have other concerns while navigating QuickBooks. I'm always here to lend a hand. Stay safe and more power to your business!