--- uc_cart_links.pages.inc
+++ (clipboard)
@@ -70,7 +70,12 @@
               // Multiple options for this attribute implies checkbox
               // attribute, which we must store as an array. However, all other
               // types can be stored as arrays as well.
-              $p['attributes'][$attribute][$option] = $option;
+              if (is_array($option)) {
+                // Already an array, just append this new option
+                $p['attributes'][$attribute][$option] = $option;
+              } else {
+                $p['attributes'][$attribute] = $option;
+              }
               break;
             // Suppress the add to cart message: m0
             case 'm':