Simplicity is the right ambition for most organisations buying a purchase order system, and it is worth being precise about which simplifications cost nothing and which quietly remove the reason to have the system. The line is clear once stated, and products marketed as simple sit on both sides of it.
Safe simplifications
No sourcing, no tendering, no contract management, no catalogues, no supplier portal, no spend analytics. These matter where somebody's job is buying and are configuration burdens everywhere else. A product without them fits a small finance team better rather than worse, and it will be adopted rather than abandoned.
The simplification that is not safe
Dropping line-level ordered, received and invoiced quantities. Without them, partial deliveries cannot be tracked, the accrual is rebuilt by hand and invoices cannot be matched properly. A product missing this is a document generator, which is a legitimate purchase but should be a deliberate one rather than a discovery in month three.
The other thing to insist on
An export you can read without the vendor, containing orders, lines, receipts, revisions and approvals. Small organisations change systems more often because they outgrow them, so this matters more here than at scale. Test it during a trial rather than accepting a description of it.
Questions people ask about simple purchase order system
How do we test line-level receipting quickly?
Order ten, receive four, invoice four, and check that six show outstanding on that line. Five minutes and it settles the question.
Is simple cheaper to run?
Usually, since configuration and training scale with surface area. That saving often exceeds any licence difference.
Will we outgrow it?
Possibly, and that is acceptable if the data comes out cleanly. Designing for leaving makes the decision low risk.