when I turn my my knob to heat, cold air still comes through the vents??

A mechanic fixed my heater core last
September. And I had heat last
winter but then in the summer time I
started having problems getting my
ac to work but it finally started
working. Now my car won't put out
heat. I took it to him and he had it for
four hours and couldn't figure out
what was. He offered to change the
thermostat to see if that might help
and charged me 75$ which is half the
original price but it did nothing so I
wasted that. I took it to another
mechanic a friend recommended to
me and within ten minutes he was
able to tell me that it's my blend door
not the actuator or "motor" because
that is moving properly but the door
is not. He said he could tell someone
had worked on my car before and
they probably caused the problem.
How much does it cost to fix this or
should this be covered under
warranty with my mechanic?