Billing: Usage record insertion fails with 'Divide by 0' or 'Attempt to de-reference a null object' in blng.UsageDomain tier pricing

Use the steps below to identify the misconfigured Order Product Consumption Schedule (OPCS) and its rates, then reload the failed Usage records.

Cause 1: Missing or zero-bound Order Product Consumption Rate (produces 'Divide by 0' at TierPricingModel.calculateUnitPrice line 198)
1. Open the failing Usage Summary record [blng__UsageSummary__c] in the browser. From the related lists, open the Order Product [blng__OrderProduct__c], then open its Order Product Consumption Schedule [blng__OrderProductConsumptionSchedule__c].
2. In the Order Product Consumption Rates [blng__OrderProductConsumptionRate__c] related list, inspect every rate. Confirm that for each rate the Lower Bound [blng__LowerBound__c] is strictly less than the Upper Bound [blng__UpperBound__c]. Lower bounds are inclusive and upper bounds are exclusive.
3. For any rate where Lower Bound equals Upper Bound, edit the rate and set the Upper Bound to the correct next-tier boundary, or delete the rate if it was created in error.
4. For the highest tier, set the Upper Bound [blng__UpperBound__c] to null to make it unbounded.
5. Confirm the Price [blng__Price__c] and Pricing Method [blng__PricingMethod__c] (Per Unit or Flat Fee) are populated on every rate. A null Price triggers the dereference error at UsageDomain.calculateSubTotalForTierUsages line 878.

Cause 2: Multiple tiers exist when only one is expected ('The order product has more than 1 tier for price schedule')
1. Navigate to the Order Product Consumption Schedule linked to the Usage Summary.
2. In the Order Product Consumption Rates related list, count the rates. If more than one rate exists but the product is configured for a single flat tier, delete the extra rates so exactly one rate remains with Lower Bound [blng__LowerBound__c] = 0 and Upper Bound [blng__UpperBound__c] = null.
3. Verify the Type [Type] field on the parent Consumption Schedule [ConsumptionSchedule] is set to either Range or Slab and matches the intended pricing behaviour.

Cause 3: Incoming usage quantity falls outside all defined tiers (produces null dereference at UsageDomain.validateTiers line 734)
1. From the failing Usage record, note the Quantity [blng__Quantity__c].
2. On the linked Order Product Consumption Schedule, confirm that the rates collectively cover the range [0, +infinity). Lower Bound of the first rate must be 0; Upper Bound of the last rate must be null.
3. Add or correct rates so the incoming quantity falls within a defined tier.

After fixing the OPCS for any cause:
1. Navigate to Setup > Installed Packages > Salesforce Billing > Configure > Usage. Confirm the Usage Rating Process Based On setting. If it is On Trigger and you are loading large volumes, set the Data Loader batch size to 1 to avoid concurrent tier evaluation issues, as documented in Salesforce Billing best practices.
2. Re-insert the failed Usage records via SOAP API or Data Loader. The Status [blng__Status__c] field on each Usage must update to Processed and the Unit Price [blng__UnitPrice__c] must inherit from the Usage Summary.
3. Confirm the issue is resolved by re-inserting one of the originally failing Usage records against the same Usage Summary and verifying that the record saves with Status = Processed and that the parent Usage Summary's Unbilled Subtotal [blng__UnbilledSubtotal__c] increments by the expected amount.