Source: https://guthlabs.ai/docs/instruments/invoice-late-fee-payment-terms-calculator/implementation/

instrument implementation / implementation-contract

# Invoice Late Fee / Payment Terms Calculator implementation

A source-bound implementation contract for Invoice Late Fee / Payment Terms Calculator; deployment and executable qualification remain separate.

Status: **VERIFIED_CLEAN**Lifecycle: **implementation-contract**

## Binding contract

This record describes what an executable surface must bind and validate. It does not claim that a public calculator, package, or production route is deployed.

**Record owner**: Guth Instruments

**Lifecycle**: verified-clean

**Evidence ID**: t5-052

**Effect**: Computation or source lookup only

## Input and validation requirements

The calculator encounters an undefined effective annualized discount cost when net_days equals discount_days. It also has a potential behavioral break at negative days_late: the raw formula returns a negative fee unless the implementation explicitly switches to an early/on-time branch and clamps the fee to zero.

## Required corrections

No binding erratum is attached to this unique-name record. Implementers must still preserve the declared units, domain, and failure behavior.

## Inspectable interface fixture

The sample below is evidence for interface and validation tests, not a production implementation.

````
{
  "inputs": {
    "balance": 1200,
    "rate_decimal": 0.12,
    "days_late": 45,
    "discount_decimal": 0.02,
    "discount_days": 10,
    "net_days": 30
  },
  "expected_outputs": {
    "late_fee_usd": 17.75342466,
    "effective_annualized_cost_percent": 37.24489796
  },
  "working": "Late fee: 1200 × 0.12 = 144; 144 × 45 = 6480; 6480 ÷ 365 = 17.75342466. Discount cost: 0.02 ÷ (1 − 0.02) = 0.02 ÷ 0.98 = 0.02040816327; net interval = 30 − 10 = 20 days; 365 ÷ 20 = 18.25; 0.02040816327 × 18.25 = 0.3724489796; 0.3724489796 × 100 = 37.24489796%."
}
````

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/invoice-late-fee-payment-terms-calculator/>)[02**Mechanism**](<https://guthlabs.ai/docs/instruments/invoice-late-fee-payment-terms-calculator/mechanism/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/invoice-late-fee-payment-terms-calculator/qualification/>)

## Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

- `VERIFIED_LEDGER.json`

- `evidence-tranche5/tranche5-results.json`