You'll pay tax on stuff over £18 @ 17.5%.
Also companies will put their "handling charge" on over here (Parcel Force is £8, UPS is £15-odd . . . )
I know it's awkward but if possible I'd suggest getting them sent either separately or in pairs to reduce weight down in order to send USPS.
That way you'll save a fortune compared to UPS/FedEx . . . etc.