Canada Post Shipping Method should supply a hook for users to filter Lettermail rates.
It would be useful if we could filter the Lettermail rates. The use cases are plenty, but two examples would be when Canada Post changes their rates (as happened today) or if the website owner would like taxes automatically applied to the rate.
At the very least the value returned by WC_Shipping_Canada_Post::calculate_lettermail_rate should be wrapped in an apply_filters call. This would allow devs to adjust the cost as well as the label and sort order.
It would also be useful if $lettermail_boxes is filtered as well for full control of the costs.
If we want to get all fancy, adding settings to the settings panel would be great so that my clients can change the rates whenever they want ;)
I'd be more than happy to contribute the code (via a secret Gist or something) or test the changes for you.