Cardinal glass company in Ocala the largest manufacture of windows in US
They make a
solar glass #320 that is amazing, look them up online
I did mine with that 320 double glazed it was under $10 a sq ft for the glass without labor.
So any installer has a low cost supplier nearby
So about $3,000 for 3 walls of a 12x20 never needed AC left the sliders out.
So $18,000 is an outrageous rip off
Do not use aluminum frames they transmit heat & cold.
Wood or vinyl for frames is better.
Be sure they have ARC approval & building permits.