The free postage related to the standard shipping charges from the Society6 website, which were indeed waived, so I would disagree that was misleading.
There is also clear information on the site here - <a class="postlink" href="http://society6.com/help/faq" onclick="window.open(this.href);return false;">http://society6.com/help/faq</a> - under the heading International Shipping.
The charge for goods above £15 (HM Revenue and Customs) is explained in the link i previously posted and is not something that is exclusive to my site. It applies to all goods bought from outside the UK. It is government policy, again not something i have any control over.
I am genuinely sorry that you have been charged by Parecelforce but if their official explanation is "theses charges are random", then i cant see how that qualifies them to ask you for money. They must be able to quantify it in some way, else its just a license to print money.
Unfortunately, this matter will have to be taken up with either Parcelforce or HMRC, I'm sorry I cant offer a better alternative but these charges have not originated from myself.
I did have a bit of a dig around for you, and it seems Parcelforce have a bit of a reputation for this kind of thing, the following link may offer some helpful leads for you -
http://psych0naut.livejournal.com/36270.html