If it's over the £18 threshold you're likely to get hit up for VAT & Royal Mail's hefty eight quid handling fee. Given that medicine is zero-rated I don't think we should be charged VAT, but unfortunately Inhouse et al insist on marking items as 'health products'. I suppose one could always visit your local delivery office and argue this point with the man behind the desk....
If that doesn't appeal, payment is in person (at your local delivery office - address will be on the card R. Mail sends as notification of the Customs charge), or on my last shipment there was an online option - you pay and then postie delivers the package. Much easier.
Slarti