

Split-screen rule editor: rules on the left, a live test cart on the right with the resulting shipping cost and free-shipping progress.
Build shipping rates from rules, and test them before they go live. The settings page is split in two: the rule builder on the left, a test cart on the right that re-evaluates your unsaved rules on every keystroke and shows exactly which condition passed or failed and how the cost was built.
Conditions
Rules
Free shipping
Testing & debugging
Pro is a separate add-on plugin. It adds 24 more conditions (postcode, city, state, products, categories, attributes, customers, payment method, coupon codes, dates, sizes), item-scoped figures, alternative (OR) condition groups, tier tables, formulas and percentage costs, rule actions (withdraw the method, rename it, delivery estimates, customer notices, hide other methods), lowest / highest / first-match combining, method minimum / maximum, saved test scenarios, load-a-real-order, a zone view, revisions, copy to zone and 26 more templates.
Everything you build in the free plugin keeps working when you add Pro. If Pro is deactivated later, rules that depend on it are kept unchanged and skipped (never silently altered) until it is active again.
The plugin is extensible without touching its files:
wbsfs_conditions, wbsfs_sanitize_condition, wbsfs_sanitize_rule, wbsfs_sanitize_cost, wbsfs_rule_cost, wbsfs_rule_effects, wbsfs_engine_matched, wbsfs_engine_combine, wbsfs_engine_final, wbsfs_form_fields, wbsfs_lint, wbsfs_extras_html, wbsfs_admin_data, wbsfs_preview_result, and more. A condition definition may carry its own evaluate callback.wp.hooks filters and actions named wbsfs.*, plus helpers on window.WBSFS_API. Add-on scripts depend on the wbsfs-admin handle; the editor boots on DOM ready, after they registered.