From HMRC
What the £135 threshold is based on
The starting point for determining whether a sale of goods located outside of the UK, made to a UK consumer, is subject to UK supply VAT under this measure is to consider whether the selling price of the goods exceeds £135, exclusive of VAT. For this purpose, the value should be based on the ‘intrinsic value’ of the goods, which equates to the price at which the goods are sold, excluding:
transport and insurance costs, unless they are included in the price and not separately indicated on the invoice
any other taxes and charges identifiable by the customs authorities from any relevant documents