Checkout, once free shipping is unlocked

WooCommerce picks a shipping method again whenever the choice of rates changes, and it picks the first one your zone lists. So in a zone that lists Flat rate above Free shipping, a shopper who crosses the threshold still has Flat rate selected at checkout, just after the bar told them shipping is free. Setup → Checkout → When free shipping is unlocked decides what happens instead:

• Leave the shopper's choice as it is. WooCommerce's own behaviour: the rate it picked, or the one the shopper chose, stays selected.
• Choose Free shipping for the shopper. When WooCommerce picks again and Free shipping is on offer, Free shipping is selected. The paid rates stay listed, so a shopper who then chooses express keeps express. A shopper who chose Local pickup keeps it too.
• Show only the free options. Once Free shipping is available, the paid rates are hidden and only Free shipping and pickup are offered. Below the threshold every rate shows as usual.

Choose Free shipping for the shopper acts when WooCommerce picks a method again, which it does when the rates on offer change. Crossing the threshold is the usual moment. A shopper who is already past the threshold with a paid rate selected when you make the change keeps that rate until their rates change again, for example when their cart or address changes. Show only the free options changes the rates themselves, so it reaches every shopper on their next cart calculation.

On WooCommerce 9.9 and later, Show only the free options is WooCommerce's own setting, Hide shipping rates when free shipping is available under WooCommerce → Settings → Shipping → Shipping settings. Choosing it here ticks that box, and choosing either of the others unticks it. Ticking the box there shows here as Show only the free options, and unticking it there while this plugin is active shows here as Leave the shopper's choice as it is. Because it is WooCommerce's setting, it goes on hiding the paid rates while the bar is off, and when Setup uses an amount you enter. To bring them back, choose another answer here or untick the box in WooCommerce. A settings file carries this answer, and importing one sets WooCommerce's box to match. On an older WooCommerce the plugin hides the paid rates itself, and only while the bar is on.

This works with the classic checkout and the Checkout block alike, and with the cart page's shipping calculator. The plugin's own part, choosing Free shipping and hiding the paid rates before WooCommerce 9.9, applies while the bar is on and reads WooCommerce's threshold. With Enter an amount chosen, free shipping comes from another plugin that this one cannot see, so the plugin leaves checkout alone and the screen says so. If WooCommerce's own box is ticked, the screen says that paid rates are still hidden instead.

A new install starts on Choose Free shipping for the shopper. A shop that had the plugin before this setting keeps Leave the shopper's choice as it is, and the Dashboard suggests the change once. It does so only while the bar reads WooCommerce's threshold and WooCommerce's own Hide shipping rates when free shipping is available is not ticked, and only when the zone your store address is in lists a paid method before Free shipping. A Flat rate that charges nothing does not count. Choose it for them makes the change; No thanks puts the suggestion away.

Uninstalling the plugin leaves WooCommerce's own Hide shipping rates when free shipping is available as it is, ticked or not: it is WooCommerce's setting.