Quick tip that could be relevant for Indian businesses - Most of the invoice recipients insist on the invoice sent on the letterhead.

While asking for an image/banner to be used as letterhead could be too 'tedious'. How about letting the user attach company logo in the email & you process it as letterhead.

Company logo & name below it on the left, company address on the right. May be - make the background color gray to show it as letterhead.