We have made some changes to the .pdf theme we'll use for sending invoices and quotes, such as presenting payment information more clearly, terms etc. However payment receipts emailed to customers also include the custom html, some of which is not relevant. Changing the pdf theme per customer doesn't resolve it as customers will receive both invoices and receipts from us. Therefore the ability to assign a specific pdf theme to a specific email template would allow far greater customization. I also wanted to add a small ad on receipts that links to a loyalty card registration form.