The return rules are too limited.

You can only turn on one carrier for all countries. But return rates vary tremendously from one carrier to another in different countries.

You should be able to turn on/off carriers through new rules and calculate rates based on:

So that a pickup of a heavy parcel from Spain is only possible if the customer pays a fair fee for it. But that a small return package from Germany remains free if the consumer chooses the carrier with the favorable rate. However, if he chooses a more expensive carrier then he too will have to pay a small fee.