Checkout customisation is one of the most consistently misunderstood parts of Shopify Plus, in both directions. Some merchants assume Plus means a fully open, rebuild-anything checkout, it doesn't. Others assume checkout is basically fixed regardless of tier, which undersells what Plus genuinely opens up. The real answer sits in between, and it's worth understanding precisely before scoping a project around it.
What's Actually Locked, Even on Plus
Shopify's checkout retains a fixed core structure across every plan, for security and PCI-DSS compliance reasons that apply regardless of tier. You can't rebuild the checkout flow from scratch or fundamentally restructure how payment is captured. This is a deliberate constraint, not a limitation to work around, and any agency proposing to bypass it entirely is proposing something that either isn't real or creates compliance risk.
What Plus Actually Opens Up: Checkout Extensibility
Checkout Extensibility is the mechanism, and it covers real, meaningful ground: custom branding matching your storefront design, custom fields for capturing additional data at checkout (a purchase order number for B2B buyers, a gift message, a delivery instruction), post-purchase upsell offers presented right after payment completes but before the confirmation page, and custom validation logic for specific business rules.
Shopify Functions: The Backend Half
Where Checkout Extensibility handles what buyers see and interact with, Shopify Functions handle backend logic invisibly plugged into checkout: custom discount calculation rules, shipping method availability based on business logic your rules engine defines, and payment method availability by customer segment or order characteristics. This is how genuinely custom business rules, the kind that used to require deep checkout.liquid hacking on older Shopify Plus tiers, get implemented cleanly today.
Where This Matters Most for B2B
Custom fields for purchase order numbers and cost centre codes, validation logic tied to company account status, and payment method restrictions by buyer type are exactly the checkout customisations most Shopify Plus B2B builds actually need, and Checkout Extensibility handles all of it within Shopify's supported, PCI-compliant framework, rather than requiring a workaround.
A useful question when scoping checkout work with an agency: "Is this a Checkout Extensibility customisation, or are you proposing something that works around Shopify's checkout constraints?" The honest answer for anything genuinely valuable should be the former. Proposals involving bypassing Shopify's checkout entirely are usually either technically unrealistic or a compliance risk not worth taking.
Performance, Not Just Possibility
Checkout Extensibility is built to minimise performance overhead compared to the older checkout.liquid customisation model, but it's not automatically fast, poorly optimised extensions, particularly ones calling external APIs mid-checkout, can measurably slow down exactly the page where speed matters most for completed conversions. Any checkout customisation work should be performance-tested specifically, not just functionally tested, before it ships.
Getting the Scope Right
Write down the specific checkout behaviour you need, not "more control over checkout" in the abstract, and get a direct answer on whether it's achievable through Checkout Extensibility and Functions. Almost everything B2B and D2C merchants actually need in practice fits within this framework; the cases that don't are rare enough to be worth a very specific, honest conversation rather than an assumption either way.
FAQ
Can you fully redesign Shopify checkout on Plus?
Not entirely, Shopify's checkout retains a fixed core structure for security and PCI compliance reasons on every plan, including Plus. What Plus opens up is meaningful customisation within that structure: branding, custom fields, post-purchase upsells, and validation logic, via Checkout Extensibility, not an unrestricted rebuild.
What is Shopify Functions and how does it relate to checkout?
Shopify Functions let developers write custom backend logic that plugs into specific checkout behaviours, discount calculation, shipping method selection, payment method availability, without touching the checkout's core code. It's how Plus merchants implement custom business rules that standard Shopify's fixed logic can't express.
Can we add custom fields to checkout for B2B purchase orders?
Yes. Checkout Extensibility supports custom fields, useful for capturing a purchase order number, a cost centre code, or other B2B-specific data at the point of purchase, which then flows through to order data for fulfilment and accounting.
Do checkout customizations slow down the checkout page?
They can, if built carelessly. Checkout Extensibility is designed to minimise performance impact compared to the older checkout.liquid customisation model it replaced, but poorly optimised custom extensions, particularly ones making external API calls during checkout, can still measurably slow the page. Performance testing on checkout customisations specifically is worth insisting on before launch.