A fallback rate appears when none of your configured methods matched the cart, but Shopify still needed a rate to let the shopper check out - so a backup rate was served instead of your intended one.
Common causes:
Destination not covered - no rate group/zone includes the customer's country, region, or ZIP.
Cart outside every range - the cart's weight or value falls outside all your methods' ranges.
Method turned off - the method that should have applied is toggled off.
No checkout matched - the shopper didn't match any segmented checkout and the Default doesn't cover their case.
What to check:
Confirm a rate group covers the order's destination.
Check the cart's weight/value against your method ranges (watch for uncapped gaps).
Verify the expected method is on and lives in the checkout that shopper matched.
💡 Share the order number with support and we can confirm from the logs exactly why it fell back.
Related: Why aren't my shipping rates showing at checkout? · Why did a customer see the wrong checkout?