Last year I used a Campbeltown bound Citylink bus to go a couple of miles to Great Western Road (with an over 60s pass). It just so happened that was the service recommended on Google maps on arriving at Buchanan Street. However this is different to a zonal ticket. What a single fare is I don't knowI would point your question at Citylink as I know they used to accept them for local journeys like Cumbernauld to Glasgow or Baillieston to Glasgow but may have changed as they are not listed on spt website. Or they have never accepted them and us drivers never cared