It is really free
As other reviewers we found the flyer in our Plovdiv Hotel and simply went to the front door of the Post Office on the central square. Vessy, our guide, was a little surprised about the number of waiting people, but she managed to keep the group together and stick to the stated 2 hours. We walked along the shopping street and at the stadium we turned to the old town. We saw most of the interesting wooden houses there and could also catch a glimpse into the yard of the Sveti Konstantin i Elena Church although it was a high church holiday.
We finished the walk on top of Nebet Tepe right in time for the beautiful sunset over the city.
And yes, the tour is really free – surely a donation is highly welcomed and well earned!