Bigcommerce doesn't give the option of setting a meta title different from post title.

bc-inject-title.js

/************************************************************************/
/* Change title & metatitle for SEO using a data attribute on blog      */
/*   posts in Bigcommerce. Embed the title within the post like this:   */
/* <span id="meta-title" data-title="This is a good SEO title!"></span> */
/************************************************************************/
$(document).ready(function() {
  var newTitle = $('#meta-title').data('title'),
      noMetaTitle = $('meta[name="title"]').length === 0,
      hasDataTitle = newTitle !== undefined,
      notACollection = $('body').hasClass('BlogPost'),
      addMetaTitle = $('head').append('<meta name="title" content="' + newTitle + '"/>'),
      pageIsBlogPost = noMetaTitle && notACollection && hasDataTitle;

  if (pageIsBlogPost) {
    document.title = newTitle; // Optional, if you prefer this for SEO or whatever
    addMetaTitle;
  }
});