docs: crypto refund destination + real-time fee assessment (#737)

## Summary

Two clarifications to the Grid concept docs, prompted by a real-time
USDC funding support case:

- **Crypto refund destination** (`Transaction Lifecycle` → Refund
Object): a new "Refund destination" subsection explains that where a
refund lands depends on how the transaction was funded. Real-time (JIT)
quotes funded with **crypto** are refunded to the customer's **internal
account balance** for that asset — not back to the source wallet —
because Grid can't verify the sending wallet belongs to the customer.
That balance is reusable via a new quote or `POST /transfer-out`.
- **Real-time fee assessment** (`Quote System` → Fees): a new "How the
variable fee is applied" subsection documents how the variable fee is
assessed for each locked side:
- Sending locked: `received = send − send × fee` → send $50 at 10 bps ⇒
receive $49.95
- Receiving locked: `send = received / (1 − fee)` → receive $50 at 10
bps ⇒ send $50.05005005…
- Plus a note on why receiver-locked divides by `(1 − fee)` (keeps the
effective rate identical across both locked sides) and how FX layers on
for cross-currency quotes.

## Test plan

- `mint broken-links` — no broken links or anchors in the edited pages
(the new `#locked-currency-side` link resolves)
- Verified the worked numbers: `50 − 50×0.001 = 49.95`, `50 / 0.999 =
50.05005005…`
- Additive-only edits to existing pages; no new pages, no navigation
changes

---------

Co-authored-by: Peng Ying <peng@lightspark.com>

Author: ls-bolt[bot]

mintlify/platform-overview/core-concepts/quote-system.mdx

@@ -350,6 +350,32 @@ Fees are deducted from the sending amount, so:
   of that quote.
 </Note>
 
+### How the variable fee is applied
+
+The variable fee (`gridApiVariableFeeRate`) is always assessed as a fraction of the **sending** amount. How that plays out depends on which side of the quote you lock (see [Locked Currency Side](#locked-currency-side)).
+
+**Sending amount locked** (`lockedCurrencySide: "SENDING"`) — the fee is taken out of the amount you send, and the recipient receives the remainder:
+
+```text
+received = send − send × fee
+```
+
+Send $50 at a 10 bps (0.10%, `fee = 0.001`) rate: `received = 50 − 50 × 0.001 = 49.95`.
+
+**Receiving amount locked** (`lockedCurrencySide: "RECEIVING"`) — Grid solves for the send amount that nets the exact received amount *after* the same fee:
+
+```text
+send = received / (1 − fee)
+```
+
+Receive $50 at 10 bps: `send = 50 / 0.999 = 50.05005005…`.
+
+<Note>
+  Receiver-locked quotes divide by `(1 − fee)` rather than adding `received × fee` so that locking the receiving side is never cheaper than locking the sending side at the same rate. Solving `send × (1 − fee) = received` for `send` keeps the effective fee rate identical across both locked sides.
+</Note>
+
+For cross-currency quotes the exchange rate applies on top of this: the fee is taken on the sending currency, then the net amount is converted at the quoted rate. The same-currency examples above (for example USDC → USD at 1:1) show the fee in isolation.
+
 ## Best Practices
 
 <AccordionGroup>

mintlify/platform-overview/core-concepts/transaction-lifecycle.mdx

@@ -181,6 +181,18 @@ When a payment fails or is cancelled, refunds are tracked in a dedicated object
 }
 ```
 
+### Refund destination
+
+Where a refund lands depends on how the transaction was funded:
+
+- **Prefunded (internal account)** — the amount is credited back to the source internal account balance.
+- **Real-time (JIT) funding via an instant fiat rail** (RTP, PIX, SEPA Instant, etc.) — the refund is returned to the originating funding instrument.
+- **Real-time (JIT) funding via crypto** (for example USDC or BTC) — the refunded amount is credited to the customer's **internal account balance for that asset**. It is **not** returned to the wallet the funds were sent from.
+
+<Note>
+  Crypto deposits are not refunded to the source wallet because Grid cannot verify that the sending wallet belongs to your customer — the funds may have originated from an exchange or another third-party address. The credited internal balance is reusable: spend it with a new quote, or withdraw it to an external wallet with `POST /transfer-out`.
+</Note>
+
 ## Webhooks
 
 Outgoing payment webhooks use the format `OUTGOING_PAYMENT.<STATUS>`. The webhook request body contains the full transaction resource.