What nobody tells first-time buyers about extra costs

What did you actually end up paying beyond the flat price for an under-construction project? I'm seriously looking at a 2BHK in Alpha II, maybe from Sunshine Infrawell. My parents worry about under-construction risks, and honestly, the hidden costs scare me. Beyond stamp duty and registration, what else should I budget for? Like interiors, PLC, other charges. How do you really check a builder's delivery risk and legal standing beyond RERA? Any advice on title verification or experiences with Sunshine Infrawell's transparency would be great. This whole process is way more complex!