anything over £18 get customs taxes. then royal mail take £8 right away no matter how much over.
customs could also hold your stuff for a while depending on how busy/arsed they are about it, so it's not quick.
having said that, if you use reputable companies,the customs people recognise their name and tend to get them through, using UPS is expemsive but good as they have their own delivery system ie planes and couriers over in the UK.
did you not see the 2 in 1 exhaust for xs250 on ebay? they are the same bike with smaller bores/carbs!!