When ordering stuff from outside the EU, the initial cost should have duty and VAT added to it, and normally a handling charge from the carrier. However, it is fairly common for items to arrive without any duty or VAT being charged even when the order value is above the threshold below which nothing is charged (£18 for VAT, £135 for duty).
Duty depends on what you are buying, where it's coming from, and even how it's described (eg a bike light could be lighting, sports equipment or bike parts). It's all on the HMRC site if you can drive it well enough. I just make a 5% allowance when considering whether to order.
VAT is 20%.
Handling charge is normally something like £8-£12, depending on carrier.
Both duty and VAT are on the full amount including shipping.
So if you pay the foreign merchant £200 for something with a duty rate of 5%, and the carrier handling charge is £10, you should allow for the possibility of receiving a demand for an extra £62 (£200 * 1.05 * 1.20 + £10). This may be pay before delivery, pay at the door, or a bill in the post later.