PrettyDamnQuick's fallback rate is shown when none of your delivery methods apply to an order. It's a per-group safety net that keeps shoppers from getting stuck at checkout with no shipping option.
(This is different from Shopify's backup rate, which only fires if PDQ can't return any rates at all - e.g. a brief app outage. Shopify applies that one automatically; see Getting your rates to show in checkout.)
Example
If your delivery methods cover every state except New Jersey, an order shipping to New Jersey would otherwise see nothing. With a fallback rate on, that order instead sees a rate with the name and at the amount you set - so the shopper can still check out.
How to turn it on
In Shipping Rate Groups, open the group, and at the top of the Delivery Methods list open the Fallback rate to edit and enter the amount you want charged and the name you want shown at checkout, and enable them for either domestic or international carts or both.
Good to know
The fallback is set per checkout group - each group can have its own.
It's a true safety net: it ignores your shipping rules and location settings by design, so it fires whenever every other method in the group returns nothing.
Want to block a destination instead of falling back? If you'd rather show Shopify's "Shipping not available" for certain orders (e.g. states you can't ship to), you deliberately turn the fallback off on a dedicated group - see [Blocking shipping to a specific state or region].