The drop shipper that BMS use normally declares a goods value that's less than you paid (AFAICS, BMS and GBK use exactly the same warehouse/dropshipper and so both do the same thing).
The shipping co will charge duty on the declared value of the goods (not normally much, just a few percent depending on category), VAT on the total of the goods declared value and the shipping cost, plus a handling fee that's usually around £10 or thereabouts (it varies from one company to another).
One thing to watch for is that BMS and GBKs dropshipper (they are effectively one and the same company as far as the source of the goods - both send identical invoices from exactly the same Chinese drop shipper) sometimes seem to make up a fictitious bike company name on the paperwork. This caused me a serious problem, as I didn't receive the FedEx duty/VAT bill at my home address (it was sent to the non-existent bike company by FedEx) and I found myself with bailiffs at the door from a debt recovery agency who had traced the parcel delivery address. The bill was only for around £40, but it seems that, at least in my case, FedEx will chase debts even this small.
It took me several months to get this resolved and make sure that it had no impact on my credit rating. FedEx refused to accept any responsibility and I was left to do all the legwork, so I'd very strongly suggest giving the shipping co a call if you've not had a bill within a couple of weeks, especially if there is a funny address on the package suggesting you may be a bike company.