Based on my experience, any land-based courier (DPD, PPL, UPS, GLS, etc) should be preferred over national postal services, and parcels over letters and small consignments that are handled through the same processing channel. Air mail is the worst as most EU countries only have one international airport that handles all incoming mail, and they have customs working in the same building. There, even correspondence inside the EU, is screened and intercepted by customs, potentially X-rayed, using K9 and perhaps other profiling techniques too (unusual weight for the size, probably). Private couriers use decentralized networks without all mail passing through one hub per country where it could be easily checked, so I think they are a vastly superior transport option for drugs. However, they perform some checks of their own, so don't sleep on proper packaging, decoy, false identity of the sender and misdirection on the receiver's end.