Add some checks on provided payment details

diff --git a/lightning-liquidity/src/lsps1/event.rs b/lightning-liquidity/src/lsps1/event.rs
index d966f8b..cdd0995 100644
--- a/lightning-liquidity/src/lsps1/event.rs
+++ b/lightning-liquidity/src/lsps1/event.rs
@@ -15,6 +15,7 @@ use super::msgs::{LSPS1ChannelInfo, LSPS1Options, LSPS1OrderParams, LSPS1Payment
 use crate::lsps0::ser::{LSPSRequestId, LSPSResponseError};
 
 use bitcoin::secp256k1::PublicKey;
+use bitcoin::Address;
 
 /// An event which an bLIP-51 / LSPS1 client should take some action in response to.
 #[derive(Clone, Debug, PartialEq, Eq)]
@@ -164,6 +165,11 @@ pub enum LSPS1ServiceEvent {
 		counterparty_node_id: PublicKey,
 		/// The order requested by the client.
 		order: LSPS1OrderParams,
+		/// The address we need to send onchain refunds to in case channel opening fails.
+		///
+		/// Please note that you can't offer onchain payments if this was not provided by the
+		/// client.
+		refund_onchain_address: Option<Address>,
 	},
 	/// If error is encountered, refund the amount if paid by the client.
 	///
diff --git a/lightning-liquidity/src/lsps1/peer_state.rs b/lightning-liquidity/src/lsps1/peer_state.rs
index 2b94f76..1b51f64 100644
--- a/lightning-liquidity/src/lsps1/peer_state.rs
+++ b/lightning-liquidity/src/lsps1/peer_state.rs
@@ -82,6 +82,12 @@ impl PeerState {
 		Ok(())
 	}
 
+	pub(super) fn get_request(
+		&self, request_id: &LSPSRequestId,
+	) -> Result<&LSPS1Request, PeerStateError> {
+		self.pending_requests.get(request_id).ok_or(PeerStateError::UnknownRequestId)
+	}
+
 	pub(super) fn remove_request(
 		&mut self, request_id: &LSPSRequestId,
 	) -> Result<LSPS1Request, PeerStateError> {
diff --git a/lightning-liquidity/src/lsps1/service.rs b/lightning-liquidity/src/lsps1/service.rs
index 459406e..478fc29 100644
--- a/lightning-liquidity/src/lsps1/service.rs
+++ b/lightning-liquidity/src/lsps1/service.rs
@@ -22,7 +22,7 @@ use super::event::LSPS1ServiceEvent;
 use super::msgs::{
 	LSPS1ChannelInfo, LSPS1CreateOrderRequest, LSPS1CreateOrderResponse, LSPS1GetInfoResponse,
 	LSPS1GetOrderRequest, LSPS1Message, LSPS1Options, LSPS1OrderId, LSPS1OrderParams,
-	LSPS1OrderState, LSPS1PaymentInfo, LSPS1Request, LSPS1Response,
+	LSPS1OrderState, LSPS1PaymentInfo, LSPS1PaymentState, LSPS1Request, LSPS1Response,
 	LSPS1_CREATE_ORDER_REQUEST_ORDER_MISMATCH_ERROR_CODE,
 	LSPS1_GET_ORDER_REQUEST_ORDER_NOT_FOUND_ERROR_CODE,
 };
@@ -326,6 +326,7 @@ where
 			request_id,
 			counterparty_node_id: *counterparty_node_id,
 			order: params.order,
+			refund_onchain_address: params.refund_onchain_address,
 		});
 
 		Ok(())
@@ -335,6 +336,9 @@ where
 	///
 	/// Should be called in response to receiving a [`LSPS1ServiceEvent::RequestForPaymentDetails`] event.
 	///
+	/// Note that the provided `payment_details` can't include the onchain payment variant if the
+	/// user didn't provide a `refund_onchain_address`.
+	///
 	/// [`LSPS1ServiceEvent::RequestForPaymentDetails`]: crate::lsps1::event::LSPS1ServiceEvent::RequestForPaymentDetails
 	pub async fn send_payment_details(
 		&self, request_id: LSPSRequestId, counterparty_node_id: PublicKey,
@@ -343,9 +347,54 @@ where
 		let mut message_queue_notifier = self.pending_messages.notifier();
 		let mut should_persist = false;
 
+		if payment_details.bolt11.is_none()
+			&& payment_details.bolt12.is_none()
+			&& payment_details.onchain.is_none()
+		{
+			let err = "At least one payment option must be provided".to_string();
+			return Err(APIError::APIMisuseError { err });
+		}
+
+		if payment_details
+			.bolt11
+			.as_ref()
+			.is_some_and(|b| b.state != LSPS1PaymentState::ExpectPayment)
+			|| payment_details
+				.bolt12
+				.as_ref()
+				.is_some_and(|b| b.state != LSPS1PaymentState::ExpectPayment)
+			|| payment_details
+				.onchain
+				.as_ref()
+				.is_some_and(|o| o.state != LSPS1PaymentState::ExpectPayment)
+		{
+			return Err(APIError::APIMisuseError {
+				err: "All payment methods must start in ExpectPayment state".to_string(),
+			});
+		}
+
 		match self.per_peer_state.read().unwrap().get(&counterparty_node_id) {
 			Some(inner_state_lock) => {
 				let mut peer_state_lock = inner_state_lock.lock().unwrap();
+
+				// Validate payment_details against the pending request before removing it,
+				// so the LSP operator can retry on failure.
+				if payment_details.onchain.is_some() {
+					let request = peer_state_lock.get_request(&request_id).map_err(|e| {
+						let err = format!("Failed to send response due to: {}", e);
+						APIError::APIMisuseError { err }
+					})?;
+					let has_refund_addr = matches!(
+						request,
+						LSPS1Request::CreateOrder(p) if p.refund_onchain_address.is_some()
+					);
+					if !has_refund_addr {
+						// bLIP-51: 'LSP MUST disable on-chain payments if the client omits this field.'
+						let err = "Onchain payments must be disabled if no refund_onchain_address is set.".to_string();
+						return Err(APIError::APIMisuseError { err });
+					}
+				}
+
 				let request = peer_state_lock.remove_request(&request_id).map_err(|e| {
 					let err = format!("Failed to send response due to: {}", e);
 					APIError::APIMisuseError { err }
@@ -357,6 +406,7 @@ where
 						let created_at = LSPSDateTime::new_from_duration_since_epoch(
 							self.time_provider.duration_since_epoch(),
 						);
+
 						let order = peer_state_lock.new_order(
 							order_id.clone(),
 							params.order,
diff --git a/lightning-liquidity/tests/lsps1_integration_tests.rs b/lightning-liquidity/tests/lsps1_integration_tests.rs
index 01c9a38..0261a08 100644
--- a/lightning-liquidity/tests/lsps1_integration_tests.rs
+++ b/lightning-liquidity/tests/lsps1_integration_tests.rs
@@ -145,8 +145,15 @@ fn lsps1_happy_path() {
 		announce_channel: true,
 	};
 
-	let _create_order_id =
-		client_handler.create_order(&service_node_id, order_params.clone(), None);
+	let refund_onchain_address =
+		Address::from_str("bc1p5uvtaxzkjwvey2tfy49k5vtqfpjmrgm09cvs88ezyy8h2zv7jhas9tu4yr")
+			.unwrap()
+			.assume_checked();
+	let _create_order_id = client_handler.create_order(
+		&service_node_id,
+		order_params.clone(),
+		Some(refund_onchain_address.clone()),
+	);
 	let create_order = get_lsps_message!(client_node, service_node_id);
 
 	service_node.liquidity_manager.handle_custom_message(create_order, client_node_id).unwrap();
@@ -157,11 +164,14 @@ fn lsps1_happy_path() {
 		request_id,
 		counterparty_node_id,
 		order,
+		refund_onchain_address: refund_addr,
+		..
 	}) = _request_for_payment_event
 	{
 		assert_eq!(request_id, _create_order_id.clone());
 		assert_eq!(counterparty_node_id, client_node_id);
 		assert_eq!(order, order_params);
+		assert_eq!(refund_addr, Some(refund_onchain_address));
 	} else {
 		panic!("Unexpected event");
 	}
@@ -339,7 +349,7 @@ fn lsps1_service_handler_persistence_across_restarts() {
 		let create_order_id = client_handler.create_order(
 			&service_node_id,
 			order_params.clone(),
-			Some(refund_onchain_address),
+			Some(refund_onchain_address.clone()),
 		);
 		let create_order = get_lsps_message!(client_node, service_node_id);