You can now reset the opened state of minecarts.

src/main/java/noobanidus/mods/lootr/entity/LootrChestMinecartEntity.java

@@ -76,6 +76,10 @@ public void setOpened() {
     this.opened = true;
   }
 
+  public void setClosed () {
+    this.opened = false;
+  }
+
   @Override
   public boolean isInvulnerableTo(DamageSource source) {
     if (this.isInvulnerable() && source != DamageSource.OUT_OF_WORLD && !source.isCreativePlayer()) {

src/main/java/noobanidus/mods/lootr/networking/CloseCart.java

@@ -0,0 +1,37 @@
+package noobanidus.mods.lootr.networking;
+
+
+import net.minecraft.network.PacketBuffer;
+import net.minecraftforge.api.distmarker.Dist;
+import net.minecraftforge.api.distmarker.OnlyIn;
+import net.minecraftforge.fml.network.NetworkEvent;
+import noobanidus.mods.lootr.networking.client.ClientHandlers;
+
+import java.util.function.Supplier;
+
+public class CloseCart {
+  public int entityId;
+
+  public CloseCart(PacketBuffer buffer) {
+    this.entityId = buffer.readInt();
+  }
+
+  public CloseCart(int entityId) {
+    this.entityId = entityId;
+  }
+
+  public void encode(PacketBuffer buf) {
+    buf.writeInt(this.entityId);
+  }
+
+  public void handle(Supplier<NetworkEvent.Context> context) {
+    context.get().enqueueWork(() -> handle(this, context));
+    context.get().setPacketHandled(true);
+  }
+
+  @OnlyIn(Dist.CLIENT)
+  private static void handle(CloseCart message, Supplier<NetworkEvent.Context> context) {
+    ClientHandlers.handleCloseCart(message, context);
+  }
+}
+

src/main/java/noobanidus/mods/lootr/networking/PacketHandler.java

@@ -29,6 +29,7 @@ public class PacketHandler {
 
   public static void registerMessages() {
     registerMessage(OpenCart.class, OpenCart::encode, OpenCart::new, OpenCart::handle);
+    registerMessage(CloseCart.class, CloseCart::encode, CloseCart::new, CloseCart::handle);
   }
 
   public static void sendToInternal(Object msg, ServerPlayerEntity player) {

src/main/java/noobanidus/mods/lootr/networking/client/ClientHandlers.java

@@ -6,6 +6,7 @@
 import net.minecraftforge.fml.network.NetworkEvent;
 import noobanidus.mods.lootr.Lootr;
 import noobanidus.mods.lootr.entity.LootrChestMinecartEntity;
+import noobanidus.mods.lootr.networking.CloseCart;
 import noobanidus.mods.lootr.networking.OpenCart;
 
 import java.util.function.Supplier;
@@ -33,4 +34,27 @@ public static void handleOpenCart(OpenCart message, Supplier<NetworkEvent.Contex
 
     ((LootrChestMinecartEntity) cart).setOpened();
   }
+
+  public static void handleCloseCart (CloseCart message, Supplier<NetworkEvent.Context> context) {
+    World world = Minecraft.getInstance().level;
+    if (world == null) {
+      Lootr.LOG.info("Unable to mark entity with id '" + message.entityId + "' as closed as world is null.");
+      context.get().setPacketHandled(true);
+      return;
+    }
+    Entity cart = world.getEntity(message.entityId);
+    if (cart == null) {
+      Lootr.LOG.info("Unable to mark entity with id '" + message.entityId + "' as closed as entity is null.");
+      context.get().setPacketHandled(true);
+      return;
+    }
+
+    if (!(cart instanceof LootrChestMinecartEntity)) {
+      Lootr.LOG.info("Unable to mark entity with id '" + message.entityId + "' as closed as entity is not a Lootr minecart.");
+      context.get().setPacketHandled(true);
+      return;
+    }
+
+    ((LootrChestMinecartEntity) cart).setClosed();
+  }
 }

src/main/java/noobanidus/mods/lootr/util/ChestUtil.java

@@ -19,6 +19,7 @@
 import noobanidus.mods.lootr.data.NewChestData;
 import noobanidus.mods.lootr.entity.LootrChestMinecartEntity;
 import noobanidus.mods.lootr.init.ModStats;
+import noobanidus.mods.lootr.networking.CloseCart;
 import noobanidus.mods.lootr.networking.OpenCart;
 import noobanidus.mods.lootr.networking.PacketHandler;
 import noobanidus.mods.lootr.tiles.SpecialLootInventoryTile;
@@ -62,8 +63,7 @@ public static void handleLootCartSneak(World world, LootrChestMinecartEntity car
     }
 
     cart.getOpeners().remove(player.getUUID());
-    // TODO: CloseCart packet
-    OpenCart open = new OpenCart(cart.getId());
+    CloseCart open = new CloseCart(cart.getId());
     PacketHandler.sendInternal(PacketDistributor.TRACKING_ENTITY.with(() -> cart), open);
   }