the order must be called in, and they only need the 24212 part of the number. They will ship USPS in a padded envelope, so shipping is pretty cheap.
And the ones they sent me require you to have the metal floor plate early versions. I have one of them, and 2 of the newer plastic floor plate mags, and the spacers won't work with them. I need to contact S&W and do some follow-up.
Here is the early thread about these.
6906 Extended Mags It seems to indicate that they sometimes ship these with the metal floor plate, so you get 2 pieces when you order 24212. And there may be a variant that IS jjust a sleeve. Confusing as heck, and clear as mud.