Using custom rewrite rules in WordPress

I am trying to create custom rewrite rules in WordPress so that I can pass data to a page based on what is in the query string.

My page works fine:
/retailers-a-z/?merchant=some_merchant
and then also
/retailers-a-z/?merchant=some-merchant&offer=some-offer

I first tried to create a rewrite rule for this in .htaccess but since realised WordPress has it’s own internal redirection database.. so after a lot of research I managed to come up with the following code… However, it is still not working.. Whenever I try to access
/retailers-a-z/some-retailer or /retailers-a-z/some-retailer/some-offer it just loads the home page.

functions.php:

function wp_raz_rules()
{
  add_rewrite_rule(
    'retailers-a-z/([^/]+)/?$',
    'index.php?merchant=$matches[1]',
    'top'
  );

  add_rewrite_rule(
    'retailers-a-z/([^/]+)/([^/]+)/?$',
    'index.php?merchant=$matches[1]&offer=$matches[2]',
    'top'
  );

  add_rewrite_tag('%merchant%', '([^/]+)');
  add_rewrite_tag('%offer%', '([^/]+)');
}
add_action('init', 'wp_raz_rules');

function wp_raz_vars($vars)
{
  $vars[] = 'merchant';
  $vars[] = 'offer';
  return $vars;
}
add_filter('query_vars', 'wp_raz_vars');

I then believe I can access them with get_query_var('merchant') and get_query_var('offer') instead of $_GET[]

Any ideas? Thanks:)