> ## Documentation Index
> Fetch the complete documentation index at: https://developer.transactbridge.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Single Pay Invoice

> Collect one-time payments for add-on features or services using a Single Pay Invoice.

At any point during an active subscription, a merchant may need to collect an additional amount for an add-on feature or service. Instead of modifying the customer's existing subscription, the merchant can use a **Single Pay Invoice** to collect this one-time amount.

A Single Pay Invoice is useful when the additional charge is independent of the customer's recurring subscription amount.

## When to Use a Single Pay Invoice

A Single Pay Invoice can be used when:

* A customer purchases an additional feature.
* The merchant provides an additional service outside the subscription.
* The customer purchases an add-on that should not become part of the recurring subscription.
* The merchant needs to collect an additional one-time amount during an active subscription.

The customer's existing subscription remains unchanged. The Single Pay Invoice is created separately for the additional charge.
<Note>To change the existing subscription with the additional charge, the merchant should update the subscription plan instead of using a Single Pay Invoice.</Note>

## Business Use Case

Consider a customer subscribed to a SaaS platform with the following plan:

| Item                         | Amount |
| ---------------------------- | -----: |
| Monthly Subscription         |   ₹999 |
| Additional Storage Add-on    |   ₹500 |
| Total One-Time Add-on Charge |   ₹500 |

The customer is already subscribed to the ₹999 monthly plan.

During the billing period, the customer requests an additional storage package. The merchant wants to collect **₹500** for this add-on but does not want the ₹500 to become part of the customer's recurring subscription.

In this scenario, the merchant can create a **Single Pay Invoice** for ₹500.

### Flow

```mermaid theme={null}
flowchart TB
    A[Customer has active subscription] --> B[Customer requests add-on]
    B --> C[Merchant calculates add-on amount]
    C --> D[Merchant creates Single Pay Invoice]
    D --> E[TB returns payment details]
    E --> F[Customer completes payment]
    F --> G{Payment successful?}
    G -->|Yes| H[Add-on activated]
    G -->|No| I[Payment remains unsuccessful]
    H --> J[Existing subscription continues unchanged]
```

### Single pay on subscription API

Merchant can use this [API](https://docs.transactbridge.com/#42bc7a92-82c8-4999-b2da-4226e1ff94a0) to create a new invoice on an ACTIVE subscription, that can be shared with a customer.