Mail transport had been exempt from road tolls from 1785 (road tolls arose on turnpike roads which had been privately built in order to recover costs and fund repairs). In 1813 this exemption was repealed in Scotland (though not England), and in compensation the Post Office was allowed to charge an additional halfpenny on mail carried in a coach with more than two wheels in Scotland.
The entire below has an example of an "Addl / 1/2" handstamp applied.