Hi rhys4,
Thanks for your question, it is a good one.
At the current moment, QuickBooks does not have the ability to customise a customer template.
You are only able to customise invoices, quotes and sales receipts.
Although I have heard many times customers ask for the ability to customise customer statements, so this is great feedback to give back to our Product Team.
Feel free to send this through by going to Settings > Feedback.
Hopefully there are more than enough requests for this new feature to be added into QuickBooks (fingers crossed).
Kristian.