Do I need Shopify Plus, or is the standard plan fine?

Most stores don't need Plus. The standard plans handle a lot of revenue perfectly well. Plus earns its price when you need specific things: checkout customization/extensibility beyond the basics, much higher API limits, wholesale/B2B features, multiple stores/expansion under one org, or the higher-touch support and automation (Flow, scripts) that high-volume operations rely on.

A rough signal: you're hitting platform limits, running serious B2B, or doing enough volume that the fee is a rounding error against what the features unlock. Until then, the savings are better spent on growth. Upgrade when a concrete limitation is costing you money, not preemptively.