How do I set a customer to receive invoices by PDF attachment?

Login to OnePosting and click 'Customers' on the top left, then click 'Find a Customer' and enter the account name or code, and click 'Load Customers'.

Then click View beside the relevant customer in the search results, and 'Edit Message Details' halfway down the customer details page, tick the 'Include Attachments as Emails' box and click 'Commit Changes'.