All countries have a different duty-free limit that varies widely. The UK is about $20, the USA is $200 and Australia is $2,000....
Any time a package is over the duty free amount, the buyer runs the risk of having customs stop their package and asses taxes & duty (it's generally hit or miss when you use USPS; packages sent through UPS & FedEx will get hit every time they are over the amount).
When that happens, there's a handling charge added as well and the buyer is billed directly by their own government, the package won't be delivered until it is paid.
So for the UK they'd be at risk for an item over $20, and the handling fee of about $5.50 (if you shipped USPS) will be charged plus VAT (like our sales tax)--I think that's at about 20%. That makes a $100 item about $125 by the time the buyer gets it. (If you shipped FedEx or UPS the handling fees are called brokerage fees and the price is much higher.)
I'd suggest the buyer check out the "Tax on goods bought from a non-EU country" here so they can see it is their own country assessing these extra costs and that you couldn't have known what they'd be in advance:
Tax on goods ordered from or bought abroad
http://www.hmrc.gov.uk/customs/post/buying.htmand you should add info to your policies and note to buyer about customs fees being the responsibility of the buyer--as they are legally, for future transactions.
Until recently, comparatively few packages going to the UK were stopped by customs--it seems since the Olympics, many more packages are getting stopped and having customs fees billed to folks receiving packages.
What to do in this situation is had to say--the fees are the buyer's responsibility, and they are supposed to know the rules for importing goods into their own country but you could choose to pay some of the fees to see if you can come to some sort of compromise so you don't lose the sale and the item doesn't have to get sent all the way back here.
Many folks here are new to international purchasing and haven't learned the ropes yet, others have purchased but never had to pay customs out of luck and are surprised when it happens the first time. Right now, a lot of those being hit seem to be UK buyers.