[eluser]Rick Jolly[/eluser]
[quote author="walesmd" date="1188266974"]
The basic rule of thumb is:
recurring function called from controller = model
recurring function called from view = helper
[/quote]
My take:
function called from view = helper
function/method called from controller = helper, library, or model
A model is for getting/setting data, whether from a database, file, feed, etc - a fairly ignorant entity. A library class has methods that share and work with its variables. I think a library would normally have more functionality than a model - it could even use models. Maybe a library might make more sense in this case - especially for the payment gateway.