Checkout customizations are a separate configuration surface from checkout offers, even though both require Shopify Plus and both live inside the same Checkout Editor activation flow — the distinction is that a customization doesn't recommend a product at all; it changes how the existing checkout looks and behaves. Brand styling (colors, corner radius, typography), trust content and custom messaging, and hiding or showing specific payment and delivery methods based on customer data all fall under this rather than under an offer.
A few customizations exist specifically to reduce checkout friction rather than to sell anything: address validation to block PO boxes where your fulfillment can't reach them, and quantity selectors added directly to checkout line items. These are configured independently of any offer and will apply even on a checkout with no active upsell offers at all.
Store compatibility is worth checking before planning customizations in detail — not every checkout setup (certain custom themes, certain existing checkout apps) is compatible with the full customization range, and that's a per-store question rather than something determined by plan tier alone.