diff --git a/info.xml b/info.xml
index 05107ca..0dcc6d2 100644
--- a/info.xml
+++ b/info.xml
@@ -17,11 +17,12 @@
   <version>7.0.0-alpha1</version>
   <develStage>alpha</develStage>
   <compatibility>
-    <ver>6.4</ver>
+    <ver>5.75</ver>
   </compatibility>
   <comments>
     Supported CiviCRM versions:
-    - CiviCRM 6.4.1+</comments>
+    - CiviCRM 5.75+ (backport)
+    - CiviCRM 6.4.1+ (originally targeted)</comments>
   <classloader>
     <psr4 prefix="Civi\" path="Civi"/>
     <psr0 prefix="CRM_" path="."/>
diff --git a/mixin/polyfill.php b/mixin/polyfill.php
new file mode 100644
index 0000000..5e2ec24
--- /dev/null
+++ b/mixin/polyfill.php
@@ -0,0 +1,102 @@
+<?php
+
+/**
+ * When deploying on systems that lack mixin support, fake it.
+ *
+ * @mixinFile polyfill.php
+ *
+ * This polyfill does some (persnickity) deduplication, but it doesn't allow upgrades or shipping replacements in core.
+ *
+ * Note: The polyfill.php is designed to be copied into extensions for interoperability. Consequently, this file is
+ * not used 'live' by `civicrm-core`. However, the file does need a canonical home, and it's convenient to keep it
+ * adjacent to the actual mixin files.
+ *
+ * @param string $longName
+ * @param string $shortName
+ * @param string $basePath
+ */
+return function ($longName, $shortName, $basePath) {
+  // Construct imitations of the mixin services. These cannot work as well (e.g. with respect to
+  // number of file-reads, deduping, upgrading)... but they should be OK for a few months while
+  // the mixin services become available.
+
+  // List of active mixins; deduped by version
+  $mixinVers = [];
+  foreach ((array) glob($basePath . '/mixin/*.mixin.php') as $f) {
+    [$name, $ver] = explode('@', substr(basename($f), 0, -10));
+    if (!isset($mixinVers[$name]) || version_compare($ver, $mixinVers[$name], '>')) {
+      $mixinVers[$name] = $ver;
+    }
+  }
+  $mixins = [];
+  foreach ($mixinVers as $name => $ver) {
+    $mixins[] = "$name@$ver";
+  }
+
+  // Imitate CRM_Extension_MixInfo.
+  $mixInfo = new class() {
+
+    /**
+     * @var string
+     */
+    public $longName;
+
+    /**
+     * @var string
+     */
+    public $shortName;
+
+    public $_basePath;
+
+    public function getPath($file = NULL) {
+      return $this->_basePath . ($file === NULL ? '' : (DIRECTORY_SEPARATOR . $file));
+    }
+
+    public function isActive() {
+      return \CRM_Extension_System::singleton()->getMapper()->isActiveModule($this->shortName);
+    }
+
+  };
+  $mixInfo->longName = $longName;
+  $mixInfo->shortName = $shortName;
+  $mixInfo->_basePath = $basePath;
+
+  // Imitate CRM_Extension_BootCache.
+  $bootCache = new class() {
+
+    public function define($name, $callback) {
+      $envId = \CRM_Core_Config_Runtime::getId();
+      $oldExtCachePath = \Civi::paths()->getPath("[civicrm.compile]/CachedExtLoader.{$envId}.php");
+      $stat = stat($oldExtCachePath);
+      $file = Civi::paths()->getPath('[civicrm.compile]/CachedMixin.' . md5($name . ($stat['mtime'] ?? 0)) . '.php');
+      if (file_exists($file)) {
+        return include $file;
+      }
+      else {
+        $data = $callback();
+        file_put_contents($file, '<' . "?php\nreturn " . var_export($data, 1) . ';');
+        return $data;
+      }
+    }
+
+  };
+
+  // Imitate CRM_Extension_MixinLoader::run()
+  // Parse all live mixins before trying to scan any classes.
+  global $_CIVIX_MIXIN_POLYFILL;
+  foreach ($mixins as $mixin) {
+    // If the exact same mixin is defined by multiple exts, just use the first one.
+    if (!isset($_CIVIX_MIXIN_POLYFILL[$mixin])) {
+      $_CIVIX_MIXIN_POLYFILL[$mixin] = include_once $basePath . '/mixin/' . $mixin . '.mixin.php';
+    }
+  }
+  foreach ($mixins as $mixin) {
+    // If there's trickery about installs/uninstalls/resets, then we may need to register a second time.
+    if (!isset(\Civi::$statics[$longName][$mixin])) {
+      \Civi::$statics[$longName][$mixin] = 1;
+      $func = $_CIVIX_MIXIN_POLYFILL[$mixin];
+      $func($mixInfo, $bootCache);
+    }
+  }
+};
+
diff --git a/paymentprocessingcore.civix.php b/paymentprocessingcore.civix.php
index 60bd5b0..3fe3059 100644
--- a/paymentprocessingcore.civix.php
+++ b/paymentprocessingcore.civix.php
@@ -130,7 +130,11 @@ function _paymentprocessingcore_civix_civicrm_config($config = NULL) {
   $extRoot = __DIR__ . DIRECTORY_SEPARATOR;
   $include_path = $extRoot . PATH_SEPARATOR . get_include_path();
   set_include_path($include_path);
-  // Based on <compatibility>, this does not currently require mixin/polyfill.php.
+
+  if (!class_exists('CRM_Extension_MixInfo')) {
+    $f = include __DIR__ . '/mixin/polyfill.php';
+    $f(E::LONG_NAME, E::SHORT_NAME, __DIR__);
+  }
 }
 
 /**
@@ -140,7 +144,6 @@ function _paymentprocessingcore_civix_civicrm_config($config = NULL) {
  */
 function _paymentprocessingcore_civix_civicrm_install() {
   _paymentprocessingcore_civix_civicrm_config();
-  // Based on <compatibility>, this does not currently require mixin/polyfill.php.
 }
 
 /**
@@ -150,7 +153,6 @@ function _paymentprocessingcore_civix_civicrm_install() {
  */
 function _paymentprocessingcore_civix_civicrm_enable(): void {
   _paymentprocessingcore_civix_civicrm_config();
-  // Based on <compatibility>, this does not currently require mixin/polyfill.php.
 }
 
 /**