Almost every collision repair costs more than the first estimate, and that is usually honest rather than otherwise. The first estimate is written by looking at a damaged car from the outside, and nobody can see through a bumper cover.
What is hiding
Behind a bumper cover sits an energy absorber, a reinforcement bar, sometimes radar or parking sensors, and mounting brackets that bend easily. Behind a damaged wing there may be an inner structure, a headlight mount, wiring or a washer bottle. None of it is visible until the outer panel comes off.
The teardown
This is why a proper shop disassembles the damaged area before committing to a plan, and documents what it finds with photographs. A teardown turns an estimate based on what is visible into one based on what is actually broken. See what happens in a teardown.
What a legitimate supplement looks like
It names the specific parts and operations being added, it comes with photographs of what was found, and it is submitted to the insurer for approval before the work is done. It is a continuation of the same documented plan, not a new number that appears at the end.
What should make you ask questions
A supplement with no photographs. Items that would have been visible from the outside at the first inspection. Work completed before approval and presented afterwards. A total that moves several times in small increments without explanation.
Why this affects how long it takes
Each supplement needs approval, and approval takes time. Then any newly identified parts have to be ordered. This is the single largest reason repairs run past the original date. See why repair times slip.