I sell records in UK and have no problem sourcing suitable packaging; I never understand why British sellers use this envelopes which are unsuitable for sending vinyl in. I use these ones which are like the European ones
http://www.defendapack.com with a couple of their card stiffeners per package makes it reasonably strong with a packaging cost of about 50p per package (if there are more than 3-4 vinyls I simply tape the packages together).
http://www.davpack.co.uk/ now sell one like Amazon mailers which I'm thinking of trying out... I had a packaging company quote me on some immensely strong box mailers and it worked out about £1.30 with a minimum order of 1000, too many for my needs and quite pricey but I can see they would be useful for people who ship a lot of high end stuff.
I have noted that Death Waltz probably have the most expensive shipping for a UK seller but often use those crappy envelopes. No idea why, it seems crazy to put so much attention into a record then sell it short at the last stage.
As for shipping cost its a fact of life that it costs money, if you don't like it don't buy it! I used to buy a lot of records from USA but these days with a 12" costing up to $25 to ship economy its hard to justify unless its something pretty rare (then you take the risk of customs charges!). I buy a lot of stuff from Jamaica these days and it is cheaper to ship than from within the UK...