If you're in the UK or indeed anywhere within the EU there are no customs fees / taxes since the goods are shipping from the UK to another EU destination. If the orders to the rest of the world also ship from the UK then you are liable for local tax / duty* if the package is inspected. This is not the supplier's liability, it's all down to your local customs & postal people.
Worth noting too, that if the package is sent by regular mail carrier, you are less likely to have it intercepted - if the package is sent by courier (UPS, DHL FedEx etc.) it is automatically declared in the destination country and you will have to pay the additional costs either on receipt or shortly after (they send you a bill).
Different countries have different maximum allowances, so if you had a previous item of a modest value ($50 for example) get through despite a full value declaration form on it, this may be because it fell below the threshold for duty and tax.
* Duty fees also depend on the category of goods involved - clothing, food, books, electronics and so on will have different tariffs.