Exactly.What makes you think that? The OP is clearly importing 300 units all in one go. He's then going to resell them here. So it's one big package, being sent by a proper courier, and with all the customs declarations etc.
No-one can actually answer the OP's question as there simply isn't enough information. I would say that the customs declaration fee is just that - an admin fee for doing the customs declaration. The OP will be responsible for paying the duty and VAT.
There are ways of shipping things so all the duty is paid up front - it's called 'DDP' (delivery duty paid) and the sending courier can help with that. I doubt it's been shipped on this basis, instead, it will probably be 'DAP' (delivered at place) which means the recipient is responsible for all fees.
International postage from the US is expensive. Costs £8-10 just to post a Blu-ray to the UK.
Even if OP got some super dirt cheap courier posting for £4 an item, that would be £1,200 to post them individually. Obviously cheaper to send in one package & pay the duty/VAT.