SD

SD usually refers to 'Special Delivery' as in Royal Mail Special Delivery, a serviced from the Post Office used within the UK to send parcels. https://www.royalmail.com/sending/uk/special-delivery-guaranteed-1pm

By default Special Delivery it insured up to £500 (and the only service that can be used from Royal Mail that is insured by them for precious metals) additional coverage can be purchased up to £2500. Delivery is guaranteed next day by 1pm from dispatch. 

Be careful ⚠️

SD is just abbreviated letters and could stand for anything, including 'Standard Delivery' it is advisable to check with the seller first to make sure it is Special Delivery, as standard Royal Mail will not be insured.