Assuming the rod is 350 US dollars and 50 for shipping, it will probably cost £400 delivered including duty (about £320 without duty)

I regularly buy my spectacles from the states, sometimes I get charged import duty sometimes I don't, luck of the draw! (Either way the total cost of my glasses is the equivalent of one lens buying in the UK - less if I don't get charged duty)

As Alfie posted above, use PayPal or Credit card (I have a Halifax Clarity Card that I use to pay in local currency which gives a better exchange rate and no credit card charges - it works with Paypal)

If you know someone living/visiting the states, it might be worth asking them to get it for you....