You can activate backup shipping rates in Shopify to be displayed in case PrettyDamnQuick is unavailable for any reason.
How to activate backup delivery methods:
In your Shopify store's settings, go to Shipping & delivery.
Under Shipping, there's a BACKUP SHIPPING RATES section.
Tick the box by Show backup rates when app or carrier rates aren't available.
That's it.
Now you've got a backup in place just in case!