Off-the-shelf software is the right answer more often than a custom development firm will typically admit. Accounting, payroll, email, and document storage are solved problems, standardized across millions of businesses, and no sensible person should pay to have them rebuilt. The question is not whether to buy software — you already do — but which problems fall outside the standard shape that purchased products assume.
A test any owner can apply without technical knowledge: describe the process to someone outside your industry. If they say "yes, everyone does it that way," buy a product. If you find yourself explaining why your business handles it differently — because of how you price, how your customers order, how your field crews work, how your regulator wants it recorded — you have found something that packaged software will fit badly. That difference is often the very thing that wins you business, and forcing it into a generic tool tends to sand it down.
A second signal is the manual work sitting between products. When staff export from one system, reshape the data in a spreadsheet, and import it into another, that spreadsheet is doing the job of software nobody has written yet. It is unmonitored, undocumented, and dependent on one person remembering the steps. The cost of that arrangement is paid weekly and invisibly, and it does not go away by buying a fourth product.
The third signal is licensing arithmetic. Per-seat pricing is excellent when you need a fraction of what a product offers and terrible when you are paying for eighty features to use three. Once an annual subscription total starts to approach the one-time cost of building exactly what you need — something you will then own outright — the comparison deserves a serious look rather than an assumption.
The honest conclusion for most businesses is a mixture. Buy the commodity systems. Build the thin, specific layer that connects them and encodes the way you actually work. That layer is usually far smaller and less expensive than owners expect, and it is where nearly all of the recovered time comes from.