docs: billing FAQ (#86)

* add docs

* tweaks

* move billing to its own section

* tweak

---------

Co-authored-by: Henry Williams <hwill12345@gmail.com>

Author: bryantanderson

fern/docs.yml

@@ -147,6 +147,8 @@ navigation:
                 path: examples/pizza-website.mdx
               - page: Python Outbound Snippet
                 path: examples/outbound-call-python.mdx
+          - page: Billing
+            path: quickstart/billing.mdx
           - page: Code Resources
             path: resources.mdx
       - section: Customization

fern/quickstart/billing.mdx

@@ -0,0 +1,71 @@
+---
+title: Billing
+subtitle: How billing with Vapi works.
+slug: quickstart/billing
+---
+
+### Overview 
+Vapi uses a credit-based billing system where credits serve as the platform's currency. Every billable action, including making calls and purchasing add-ons, requires credits. One credit equals one US dollar ($1 = 1 credit).
+
+Each new trial account receives 10 complimentary credits in their wallet to help you get started. These credits can be used for making calls, but not purchasing add-ons.
+
+To continue using Vapi after your trial credits are depleted, you'll need to:
+1. Add your payment details
+2. Purchase additional credits
+3. Your wallet will then be upgraded to pay-as-you-go status.
+
+<Warning>
+  Your wallet will be marked as frozen while your credit balance is negative. Frozen wallets are unable to make calls or purchase add-ons. 
+</Warning>
+
+### What is auto-reload?
+
+Auto-reload is a billing mechanism in which Vapi will automatically top up your credits when it hits a certain threshold. It can be enabled through the [billing page](https://dashboard.vapi.ai/org/billing) and we highly recommend enabling it to prevent billing-related operational issues.
+
+<Note>
+  Auto-reload amounts must be at least \$10. We recommend setting a threshold above \$0.
+</Note>
+
+<Frame caption="Enabling auto-reload through the Vapi dashboard.">
+  <img src="../static/images/quickstart/billing/auto-reload.png" />
+</Frame>
+
+### What are add-ons?
+Add-ons are platform enhancements that you may purchase. They are purchased through the [billing add-ons page](https://dashboard.vapi.ai/org/billing/add-ons). 
+
+The following add-ons are currently available:
+| Add-on | Price (credits / month) | Description |
+|--------|-------------------------|-------------|
+| Phone Numbers | 2 | Twilio phone numbers for your calls |
+| Reserved Concurrency Lines | 10 | Guaranteed capacity for concurrent calls |
+| HIPAA Compliance | 1,000 | HIPAA Compliance and a BAA agreement |
+| Slack Support | 5,000 | Priority support via dedicated Slack channel |
+
+Here's how the billing works:
+- You'll be charged a prorated amount for the remainder of the current billing cycle when you first purchase an add-on
+- Subsequently, you'll be billed the full amount at the start of each billing cycle (the 1st of each month)
+- If you cancel an add-on, you'll receive a prorated refund for the unused portion of the billing cycle. 
+- Add-ons can be cancelled at any time through the billing page
+
+<Warning>
+  If your wallet is frozen at the time of billing, all add-ons will be automatically cancelled. You'll need to repurchase them once your wallet is reactivated.
+</Warning>
+
+### How do I download invoices for my credit purchases?
+
+You may download invoices within the [billing page](https://dashboard.vapi.ai/billing). 
+
+Find the relevant credit purchase within the payment history table, and click on **Download Invoice**. You may then optionally choose to fill in extra details which will be reflected within the Invoice.
+
+<Frame caption="Downloading an invoice.">
+  <img src="../static/images/quickstart/billing/download-invoice.png" />
+</Frame>
+
+<Frame caption="Optional invoice details.">
+  <img src="../static/images/quickstart/billing/invoice-detail-form.png" />
+</Frame>
+
+<Frame caption="Sample invoice.">
+  <img src="../static/images/quickstart/billing/sample-invoice.png" />
+</Frame>
+