We are loading a shipping plugin from wp-content/plugins directory instead of the shipping-plugins directory because we have read that placing plugins there can cause wordpress to delete them when the user upgrades MarketPress.
We are using the hook:
mp_load_shipping_plugins – to load our plugin.
But in your marketpress.php file, function load_shipping_plugins() (line 396)
Processes the shipping settings immediately after the function header and loads the external plugins after processing the filter, that is why we are unable to validate input data before it is being saved.
I think a simple reordering of the apply_filter block can fix this issue. Please suggest.