Had an absolute bugger of a time tracking this down but something for anyone who's mistakenly messed with their postage/shipping calculations in Paypal.
You MUST set this setting in order for your shipping information to be passed on:
QuoteClick here to allow transaction-based postage values to override the profile postage settings listed above (if profile settings are enabled).
It can be found under your paypal account in:
Selling Preferences > Postage Calculations
Most people won't have been in there but Paypal in their infinite wisdom have made it such that you can't disable or delete your settings once you've enabled them for any currency.. Instead it just zero rates anything. Idiotic but there you go.