Hi garreth!

In QuickBooks Online, you can add letters as a suffix or prefix to the invoice number if you want to set your own numbering sequence. If you added this to your previous invoices, this will be added automatically to future ones. 

You can simply edit the Invoice no. field and remove the letter from there. This will prevent from adding to future invoices.

If you're unable to edit it or you can't see the said field, let's turn on the custom transaction numbers. Please follow these steps:

1. From the Gear icon, select Account and settings.
2. Go to the Sales tab and select the Sales form content section to expand it.
3. Toggle the Custom transaction numbers to turn it on. Then, tap Save and Done.

Once done, create an invoice and remove the letter before saving it. If everything looks good, turn off the Custom transaction numbers again. Just follow the same steps.

I've added some links below for more details about customising sales forms:

• How to set up custom invoice numbers
• Email and customise sales form

I'll be here if you need anything else. Have a great day!