Hi dave-306-live-co, thanks for joining us on the Community - you can enable the letterhead heading by signing in on the web and clicking the cog > custom form styles > edit > edit print settings > tick 'use letterhead paper'. This applies to printing only, so if you'd prefer to send this online, you would instead need to enter the bank details on the footer, by selecting the content tab on the invoice template > footer > add payment details and footer.
Thanks!
