Or they could just add the LARGE parcel option under the sizes of LL small and medium and let you chose custom seeing as SD didn’t cover large or heavy parcels 

why is it stopping at medium it didn’t used to even after SD was introduced 

it might be to do with them wanting SD to cater for large parcels too with DHL now being brought on board SOON

but they have / had  nothing to lose by allowing custom for those parcels until they do so I don’t get it