Basically, the only other elegant work around is to pay another $15/mo giving you 8 mule alts (with 2 retainers each) and also allowing you to mail stuff to your other characters on your primary account by mailing to alts and then back again, giving you a total of 32 retainers at your disposal and 640 total market slots.

That's... Kinda overkill. We shouldn't have to do that.