Can I assign a custom form invoice to a specific customer?

Hello Chris-booth, 

Welcome to the Community page, 

So what you can do is create a template in the custom forms section in the main cog wheel and you would be best naming it specifically so you know which one you need. once you have created the template and saved it then when you go to create an invoice section and clicking on customise you will then just choose that template you created for them and then send it off to the customer. You will need to select that template each time for them otherwise it will always sent the defaulted one to them instead.