The duty applies to both the value of goods and delivery charges, then you apply VAT on top. Then there’s a nominal handling charge of £12-15. I’ve imported bike stuff from USA in last couple of months and it arrived without any charges to pay. I think since brexit, HMRC are a bit overwhelmed with workload.
If I remember I’ll have a look at the charges for a bike item I bought from USA in 2019, where I did pay duty and vat. When you get the invoice it’s all broken down for you.