Daniel - a complex solution to the problem would be to petition your democratically-elected government to rescind VAT and duties on foreign-made playing cards. It won't solve the shipping issue but it will make your shipments from the US more affordable.
Bhong - Anything faster than plain, ordinary First Class mail sent internationally can be prohibitively expensive when using the US Postal Service. Few US card companies will send by International Priority Mail and the ones that do charge through the nose for it.
A Priority Mail Small Flat Rate Box can hold perhaps a half-dozen decks if you stack them well. To Canada, it costs $20.55 in-store or $19.50 if ordered online. The same box to the UK is $24.75/$23.50. Send it to Australia (or most other countries, including Mexico, our neighbor to the south) and it runs the same as to the UK. It generally takes 6 to 10 business days and you can ship a maximum of four pounds, definitely more than any six packs of poker-sized playing cards I've ever seen.
The same decks sent by first-class mail in a one-pound package (no flat rate option available) is $10.50/$9.50 to Canada, $16.75/$15.08 to most of the rest of the world. This is the more commonly-used option. There's also the matter of purchasing (or re-using) packaging, since it's not provided for First Class as it is for the other services, which have a Flat Rate option based on size more than weight.
The same decks sent in a Priority Mail Express (formerly known as Express Mail) Flat Rate Padded Envelope will cost $35.95/$33.00 to Canada and $46.50/$42.75 to most of the rest of the world. Takes half as long as Priority Mail.
Also, bear in mind that none of these include the cost of the employee who tapes the box, fills it with packing material and addresses it to you...