I don't know for sure. I'm only guessing here but they are using different Postal Services, even if the parcel originate from the same geogrpahic location. UPS Air Service, for example, 'guarantee' delivery of parcel by a set date and thus cost more to ship. RK may be using Registered Air Mail Parcel, which keeps cost down but takes a lot longer to reach it's final destination. The exception is for large parcels which require EMS shipping method.