Someone else asked a similar question - see here Invoice Prefixes
Sounds like the answer is currently No.

Hi Paul

As a workaround you could add a text box to your layout with the alpha characters you'd like to display on your invoices.

You wouldn't be able to search for this full string in the invoice module though.

Kind regards

Bec

Managing Director

Sage Accounts Solutions Limited

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www.sageaccountssolutions.co.uk

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Thanks Bec, that's a very good idea - that will work well for us. Also the searching issue won't be a problem as we mostly search by customer.