As choco says, on stuff from outside the EU, tax is added at this end - you shouldn't have to pay any US tax as it's an international order (but that's not always the case).
So - if they are £70, and postage is £10 (say) that gives you £80 that's subject to 20% VAT (I know children's stuff is 0%, but it's likely they'll see "shoes" on the declaration, and at best, you'll have to pay first and claim it back from HMRC if you can). 20% of £80 is £16, plus the handling fee - £8 if it's RM, and I think £13 for DHL.
So on to the original cost and postage (in this example, £80 total, because someone said they are £68 so I rounded that up to £70 and added £10 for postage as a very rough conversion as the cheapest international postage for one pair looks like it's $14) you have to add £24 - or £29 (plus additional shipping cost) if you choose the DHL delivery option.
This means that to have your pair of shoes delivered to you in the UK from a US store, the total cost to you will be around £104 as a minimum.
It seems unfair they include the cost of your shipping (and any insurance also) in the total when assessing the tax, and that the shipper at this end also places a cost on collecting that tax for the government, but them's the breaks, unfortunately.
From memory, when they arrive, you get a card from RM telling you how much you owe, and you can pay online and then arrange for it to be delivered, or go to the delivery office on the card, pay there and collect it. It's been a couple of years since I bought from the US, and the experience put me off a bit - I knew about the costs in advance, but the item I'd ordered from the internet was not as nice as it had looked (is it ever 
?) but wasn't worth sending back, once I'd taken into account the postage, postal insurance and the faff of reclaiming (assuming I was successful) from HMRC.