What the quote usually leaves out
A build quote covers writing the software. It rarely covers hosting, backups, the GST rule that changes next year, the Android version that stops supporting something, the developer who becomes unavailable, or the six months of finding bugs that only appear with real bills. Ready software spreads all of that across thousands of shops. Custom software puts every rupee of it on you, and the ongoing part is usually larger than the build.
The dependency nobody plans for
Custom software ties your business to whoever wrote it. If that person moves on, raises their rate, or simply stops answering, you are left with code nobody else wants to touch and a shop that cannot bill. This is the single most common way custom projects end badly, and it has nothing to do with the quality of the original work.
When custom genuinely is the answer
If you have a process no product supports and that process is your actual advantage — an unusual manufacturing flow, a pricing model nobody else uses, an integration with a specific customer's system — then build it, and budget for maintaining it. Also build if you are large enough to employ or retain a developer properly. What does not work is commissioning a bespoke system because a ready one is missing two small things.
The middle path most shops take
Start on ready software and find out what you actually need, because the list you would hand a developer today is not the list you would hand them after six months of real use. Almost everything shops think is unique — their bill format, their units, their credit terms — turns out to be configuration. Whatever genuinely remains is a much smaller and better-specified problem than the one you would have paid to solve at the start.