Definitely don't use standard international post - Corsair have a horrible record of losing RMA RAM and then challenging the customer to prove it was delivered.
If you're looking at a pair of sticks that now retail at £35, then they may well be borderline in terms of whether it's worth RMAing to the US. Royal Mail Airsure (simple confirmation of delivery service) would take 4 days to get there and would cost not more than £9.92 according to their online calculator or you can use International Signed For Small Packet Rate which takes 5 days and gets you a signature at the other end for £9.22. Personally, I'd opt for the signed-for service as you know who signed for it!
Don't get me started on Corsair PSU postage costs!