I don't know how many US-based customers you have, but I'm guessing it's a measurable proportion of your total. Your service is much more 'international traveler friendly' than most UK service providers I've tried, to put it mildly.
It can take a while for airmail to reach a US address (sadly the US Postal Service doesn't meet the high standard set by the Royal Mail). I'm in California, so not quite worst case (that's probably Hawaii and Alaska) but still, on the upper end of your shipping estimates.
Have you considered engaging with a US-based fulfillment house? less international postage cost for you, quicker delivery for your U.S. customers.