A ready-made package is the right choice when your work resembles the flow the package assumes. If invoices are simple, stock is simple, and exceptions are rare, writing new software wastes time.
Custom software is needed when the exception becomes the rule. Dealer prices change, a quote is revised three times, the service note stays in WhatsApp, or someone copies rows between two systems every day.
Do not choose custom software because it sounds prestigious. Write down, for a week, where the team stops. If the stop is always the same step, the software is designed for that step. If the stops are everywhere, simplify the process first. Code does not speed up a messy process. It makes the mess permanent.
In discovery we say this distinction at the start. If a package is enough, we say that too. If it is not, we phase the work and solve the step that eats the most time first.