Create a custom Postageapp mailer class as follows (you can also copy the default app/mailers/devise/mailer.rb file in the devise library and modify it accordingly):
In the /config/initializers/devise.rb file, set the custom mailer to be:
config.mailer = "MyDeviseMailer"
Our good friend, Mark Foster, pointed out that emails sent through Devise without subject lines specified would send out the Devise defaults. As such, he has sent us an addition to his original Devise code (which we have updated above) that sends a blank subject line to the PostageApp API, which uses the templates you have specified in your backend.