# API Reference

When you send an invoice to a customer, the invoice moves from draft to payable state. PayPal then emails the customer a link to the invoice on the PayPal website. Customers with a PayPal account can log in and pay the invoice with PayPal. Alternatively, customers can pay as a guest with a debit card or credit card. 

For more information, see the [Invoicing Overview](/online-payments/invoicing/integration-guide.md) and the [Invoicing Integration Guide](/online-payments/invoicing/integration-guide.md).

{% content-ref url="/pages/70096dfc61d0e9ab4d38b85e1ff0491a110aab9b" %}
[Invoices](/online-payments/invoicing/api-reference/invoices.md)
{% endcontent-ref %}

{% content-ref url="/pages/4515d96c22f04dc567caf2c41e0082ddcf3e1a35" %}
[Search Invoices](/online-payments/invoicing/api-reference/search-invoices.md)
{% endcontent-ref %}

{% content-ref url="/pages/eeced1e2a319c364a3c3bd2c7fe9d0a1e414dff0" %}
[Templates](/online-payments/invoicing/api-reference/templates.md)
{% endcontent-ref %}


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# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://paypal.gitbook.com/online-payments/invoicing/api-reference.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.