diff --git a/app/assets/javascripts/shopify_app/bottle-deposit-sync.js b/app/assets/javascripts/shopify_app/bottle-deposit-sync.js
new file mode 100644
index 000000000..82aaf8163
--- /dev/null
+++ b/app/assets/javascripts/shopify_app/bottle-deposit-sync.js
@@ -0,0 +1,233 @@
+(function () {
+  if (!window.fetch || !window.Promise) {
+    return;
+  }
+
+  var config = window.BC_DEPOSIT_SYNC;
+
+  if (!config || !config.depositHandle || !config.variantIdByValue) {
+    return;
+  }
+
+  var depositHandle = config.depositHandle;
+  var variantIdByValue = config.variantIdByValue;
+  var isSyncing = false;
+  var syncQueued = false;
+  var originalFetch = window.fetch.bind(window);
+
+  function normalizeDepositValue(value) {
+    if (value === null || value === undefined) {
+      return null;
+    }
+
+    var normalized = String(value).trim();
+    return normalized.length ? normalized : null;
+  }
+
+  function parseContainersPerUnit(value) {
+    var parsed = parseInt(value, 10);
+    return isNaN(parsed) ? null : parsed;
+  }
+
+  function extractUrl(resource) {
+    if (typeof resource === "string") {
+      return resource;
+    }
+
+    if (resource && typeof resource.url === "string") {
+      return resource.url;
+    }
+
+    return "";
+  }
+
+  function isCartMutation(resource) {
+    var url = extractUrl(resource);
+    if (!url) {
+      return false;
+    }
+
+    try {
+      url = new URL(url, window.location.origin).pathname;
+    } catch (error) {
+      return /\/cart\/(add|change|update)\.js/.test(url);
+    }
+
+    return /\/cart\/(add|change|update)\.js/.test(url);
+  }
+
+  function queueSync() {
+    if (syncQueued) {
+      return;
+    }
+
+    syncQueued = true;
+    setTimeout(function () {
+      syncQueued = false;
+      syncDeposits();
+    }, 0);
+  }
+
+  function fetchJson(path, options) {
+    var fetchOptions = {
+      credentials: "same-origin",
+      headers: {
+        "Content-Type": "application/json",
+        Accept: "application/json",
+      },
+    };
+
+    if (options) {
+      Object.keys(options).forEach(function (key) {
+        fetchOptions[key] = options[key];
+      });
+    }
+
+    return originalFetch(path, fetchOptions).then(function (response) {
+      return response.json();
+    });
+  }
+
+  function fetchCart() {
+    return fetchJson("/cart.js");
+  }
+
+  function changeLineItem(key, quantity) {
+    return fetchJson("/cart/change.js", {
+      method: "POST",
+      body: JSON.stringify({
+        id: key,
+        quantity: quantity,
+      }),
+    });
+  }
+
+  function addVariant(variantId, quantity) {
+    return fetchJson("/cart/add.js", {
+      method: "POST",
+      body: JSON.stringify({
+        id: variantId,
+        quantity: quantity,
+      }),
+    });
+  }
+
+  function buildRequiredDeposits(items) {
+    return items.reduce(function (required, item) {
+      if (item.handle === depositHandle) {
+        return required;
+      }
+
+      var properties = item.properties || {};
+      var depositValue = normalizeDepositValue(properties._deposit);
+      var containersPerUnit = parseContainersPerUnit(properties._containers_per_unit);
+
+      if (!depositValue || !containersPerUnit || containersPerUnit <= 0) {
+        return required;
+      }
+
+      var variantId = variantIdByValue[depositValue];
+      if (!variantId) {
+        return required;
+      }
+
+      var total = item.quantity * containersPerUnit;
+      required[variantId] = (required[variantId] || 0) + total;
+      return required;
+    }, {});
+  }
+
+  function buildDepositUpdates(items, requiredByVariantId) {
+    var updates = [];
+
+    items.forEach(function (item) {
+      if (item.handle !== depositHandle) {
+        return;
+      }
+
+      var desiredQuantity = requiredByVariantId[item.variant_id] || 0;
+
+      if (item.quantity !== desiredQuantity) {
+        updates.push({
+          key: item.key,
+          quantity: desiredQuantity,
+        });
+      }
+
+      delete requiredByVariantId[item.variant_id];
+    });
+
+    return updates;
+  }
+
+  function buildDepositAdds(requiredByVariantId) {
+    return Object.keys(requiredByVariantId)
+      .map(function (variantId) {
+        return {
+          id: variantId,
+          quantity: requiredByVariantId[variantId],
+        };
+      })
+      .filter(function (entry) {
+        return entry.quantity > 0;
+      });
+  }
+
+  function applyUpdates(updates, adds) {
+    var sequence = Promise.resolve();
+
+    updates.forEach(function (update) {
+      sequence = sequence.then(function () {
+        return changeLineItem(update.key, update.quantity);
+      });
+    });
+
+    adds.forEach(function (entry) {
+      sequence = sequence.then(function () {
+        return addVariant(entry.id, entry.quantity);
+      });
+    });
+
+    return sequence;
+  }
+
+  function syncDeposits() {
+    if (isSyncing) {
+      return Promise.resolve();
+    }
+
+    isSyncing = true;
+
+    return fetchCart()
+      .then(function (cart) {
+        var items = (cart && cart.items) || [];
+        var requiredByVariantId = buildRequiredDeposits(items);
+        var updates = buildDepositUpdates(items, requiredByVariantId);
+        var adds = buildDepositAdds(requiredByVariantId);
+
+        if (!updates.length && !adds.length) {
+          return null;
+        }
+
+        return applyUpdates(updates, adds);
+      })
+      .catch(function () {})
+      .then(function () {
+        isSyncing = false;
+      });
+  }
+
+  window.fetch = function () {
+    var args = arguments;
+    return originalFetch.apply(window, args).then(function (response) {
+      if (!isSyncing && isCartMutation(args[0])) {
+        queueSync();
+      }
+
+      return response;
+    });
+  };
+
+  document.addEventListener("DOMContentLoaded", queueSync);
+  queueSync();
+})();
diff --git a/lib/shopify_app/engine.rb b/lib/shopify_app/engine.rb
index fe5cc2e52..93b143c55 100644
--- a/lib/shopify_app/engine.rb
+++ b/lib/shopify_app/engine.rb
@@ -19,6 +19,7 @@ class Engine < Rails::Engine
     initializer "shopify_app.assets.precompile" do |app|
       app.config.assets.precompile += [
         "shopify_app/redirect.js",
+        "shopify_app/bottle-deposit-sync.js",
       ]
     end