You call all the passenger transport companies, try to talk to as many drivers as possible, find out who the most corrupt customs officers are and you know how much to pay, simple as that, bro.
NL and Spain are really not the best SCHENGEN countries to order from !!!
They're the most closely watched in Europe!
I order from Germany, and never had any problem (make sure the seller does his job properly and sends secure, discreet, kevlar-wrapped shipments).
Why spain? Thought NL and Belgium were closely watched within schengen. Is there any reliable data available on this? I searched far and while ago and couldn't find anything besides NL and anecdotes
Belgium is monitored for large-scale arrivals (from other continents : especially South America and Asia) at the port of Antwerp (which are immediately dispatched to the rest of Europe). So if you're not planning to buy one or two tons, Belgium should be safe.
Spain and the Netherlands are two of Europe's best-known countries for buying “substances” in small quantities (up to a few kilos). All the go-fasts that get busted in Europe come from there.
Why Spain? Because it's the closest European country to North Africa (14 km by sea), and even has colonies there: Ceuta and Melilla. But the smugglers only manage to get small quantities across, because it's heavily guarded against illegal African migrants.
Numerous reliable data (by the authorities) are available on this subject, published in the language(s) of the country publishing them and not in English (no Schengen country has English as its national language). So you're going to have to get into Schengen languages if you want to find them, and I'm not going to do the job for you (even if I speak five of those languages) !
Safety is a big scale so what are you afraid of ?