This article concerns clients using using idev_OneStepCheckout. For incompatibility OneStepCheckout see

incompatible OneStepCheckout extension


In the backend configuration of the webshop:

System > Configuration > Sales > OneStepCheckout > General > Enable different shipping address -> this needs to be set to "YES"

The checkout quote has to contain a shipping address, disabling this option removes the shipping address from quote, breaking the bpost module.