Please note: All mechanics everywhere are cheaters and scams. For Screen issues, there are high chances that there is no Panel problem. (It could be due to voltage issues, back light issues or small connection problems internally). But, mechanics loot money from consumers just for doing simple tasks.
Here are few tips:
(a) If you are good in hardware opening, then, follow various YouTube videos and check. (Provided, you have few equipments like voltage checking, shouldering machine, etc.)
(b) If you know a mechanic who knows you well or a relative, it helps a lot. At least, you can trust him to check the small things.
(c) Raise a complaint against the mechanic / service centre to the manufacturer about their cheating attitude.