Include invoice_slot in OfferPathsRequest message

diff --git a/lightning/src/offers/async_receive_offer_cache.rs b/lightning/src/offers/async_receive_offer_cache.rs
index 1b1078d..b3e6b83 100644
--- a/lightning/src/offers/async_receive_offer_cache.rs
+++ b/lightning/src/offers/async_receive_offer_cache.rs
@@ -246,10 +246,11 @@ impl AsyncReceiveOfferCache {
 			.ok_or(())
 	}
 
-	/// Remove expired offers from the cache, returning whether new offers are needed.
+	/// Remove expired offers from the cache, returning the first slot number in the cache that needs
+	/// a new offer, if any exist.
 	pub(super) fn prune_expired_offers(
 		&mut self, duration_since_epoch: Duration, force_reset_request_attempts: bool,
-	) -> bool {
+	) -> Option<u16> {
 		// Remove expired offers from the cache.
 		let mut offer_was_removed = false;
 		for offer_opt in self.offers.iter_mut() {
@@ -268,8 +269,14 @@ impl AsyncReceiveOfferCache {
 			self.reset_offer_paths_request_attempts()
 		}
 
-		self.needs_new_offer_idx(duration_since_epoch).is_some()
-			&& self.offer_paths_request_attempts < MAX_UPDATE_ATTEMPTS
+		if self.offer_paths_request_attempts >= MAX_UPDATE_ATTEMPTS {
+			return None;
+		}
+
+		self.needs_new_offer_idx(duration_since_epoch).and_then(|idx| {
+			debug_assert!(idx < MAX_CACHED_OFFERS_TARGET);
+			idx.try_into().ok()
+		})
 	}
 
 	/// Returns whether the new paths we've just received from the static invoice server should be used
diff --git a/lightning/src/offers/flow.rs b/lightning/src/offers/flow.rs
index b6eee42..7c809ad 100644
--- a/lightning/src/offers/flow.rs
+++ b/lightning/src/offers/flow.rs
@@ -1266,11 +1266,11 @@ where
 
 		// Update the cache to remove expired offers, and check to see whether we need new offers to be
 		// interactively built with the static invoice server.
-		let needs_new_offers =
-			cache.prune_expired_offers(duration_since_epoch, timer_tick_occurred);
-		if !needs_new_offers {
-			return Ok(());
-		}
+		let needs_new_offer_slot =
+			match cache.prune_expired_offers(duration_since_epoch, timer_tick_occurred) {
+				Some(idx) => idx,
+				None => return Ok(()),
+			};
 
 		// If we need new offers, send out offer paths request messages to the static invoice server.
 		let context = MessageContext::AsyncPayments(AsyncPaymentsContext::OfferPaths {
@@ -1289,7 +1289,9 @@ where
 
 		let mut pending_async_payments_messages =
 			self.pending_async_payments_messages.lock().unwrap();
-		let message = AsyncPaymentsMessage::OfferPathsRequest(OfferPathsRequest {});
+		let message = AsyncPaymentsMessage::OfferPathsRequest(OfferPathsRequest {
+			invoice_slot: needs_new_offer_slot,
+		});
 		enqueue_onion_message_with_reply_paths(
 			message,
 			cache.paths_to_static_invoice_server(),
diff --git a/lightning/src/onion_message/async_payments.rs b/lightning/src/onion_message/async_payments.rs
index 52badd7..1582543 100644
--- a/lightning/src/onion_message/async_payments.rs
+++ b/lightning/src/onion_message/async_payments.rs
@@ -131,7 +131,13 @@ pub enum AsyncPaymentsMessage {
 ///
 /// [`Offer::paths`]: crate::offers::offer::Offer::paths
 #[derive(Clone, Debug)]
-pub struct OfferPathsRequest {}
+pub struct OfferPathsRequest {
+	/// The "slot" in the static invoice server's database that this invoice should go into. This
+	/// allows us as the recipient to replace a specific invoice that is stored by the server, which
+	/// is useful for limiting the number of invoices stored by the server while also keeping all the
+	/// invoices persisted with the server fresh.
+	pub invoice_slot: u16,
+}
 
 /// [`BlindedMessagePath`]s to be included in an async recipient's [`Offer::paths`], sent by a
 /// static invoice server in response to an [`OfferPathsRequest`].
@@ -233,7 +239,9 @@ impl OnionMessageContents for ReleaseHeldHtlc {
 	}
 }
 
-impl_writeable_tlv_based!(OfferPathsRequest, {});
+impl_writeable_tlv_based!(OfferPathsRequest, {
+	(0, invoice_slot, required),
+});
 
 impl_writeable_tlv_based!(OfferPaths, {
 	(0, paths, required_vec),