From a few years' of experience ordering rolls:

UK duty is 5% on the cost of the rolls (they count as recordings, and
are listed in the Customs man's book).  17.5% VAT (sales tax European
style) is charged on the total value of rolls plus postage plus duty. A
service charge of about $7 per parcel is added.  So, rolls costing $100
with post of (say) $20 will be charged 5 + (105 + 20) * 0.175 + 7 =
$34.  So, $100 of rolls costs $154.  Ouch!

Beware the mathematics.  I was once charged on the dollar amount as if
was UK pounds -- an overcharge of about 50%.  Refunded instantly, but
still annoying.

The UK customs cutoff is rather flexible and unpredictable -- probably
US$100-$150 averaged over time.  The cutoff seems to apply to the value
of the goods, not the postage.  The secret then is to keep the value
per package of the goods low.  Packing and declaration is up to
vendors, who don't get hit for duty.  Vendors dealing overseas should
learn the ropes to do the best for their customers and gain repeat
sales!

An interesting approach I've seen is to declare rolls as 'sheet music',
which then counts as books, which are not dutiable (yet).  Vendors
sending books or printed matter should always declare it as such,
and look for the special postage rates such material offers.

Get the postage rates book, and don't rely on the counter clerk who
may not know all the special rates.  Be prepared to argue.  I saved
UKP30 once on a parcel by persisting until they got it right, and it
took about five goes until they came up with the cost I had worked out
at home!

Julian Dyer