Does the process differentiate you, or is it just yours?
Every company believes its process is unique. Most of the time it is merely familiar. The test is whether a customer would notice if you did it the standard way. If invoicing works oddly because of a decision made in 2019 that nobody has revisited, that is not differentiation, it is inertia, and encoding it in custom software makes it permanent. If the process is genuinely how you win work, custom is defensible.