I think I understand the issue here. Originally you would purchase the Security Token through the PlayOnline viewer, yes? That is no longer available, but there is a new method now.
The new method for purchasing a security token is not immediately obvious, it takes a bit of poking around to find it.
What you do is:
A) Log in at
https://secure.square-enix.com/account
A1) Select "One-Time Password" under 'Services and Options' on the left side of the screen. (You may need to scroll down to see it.)
B) You should now be at the URL
https://secure.square-enix.com/account/app/svc/accSvc. Here you will see a link to read about one-time passwords and two options: "Security Token Usage Settings" and "Purchase a Square Enix Security Token".
B1) Select "Purchase a Square Enix Security Token" and click 'Next'.
B2) Now, read through and accept the Security Token Terms of Use agreement (or scroll to the bottom of it if you can't be bothered to know what you're agreeing to), check the 'I accept' box, and click 'Next'.
C) You're now looking at a screen that will say the price of the Security Token and ask you to select your country.
C1) After you select your country, input your shipping information. You should probably double-check and make sure you didn't mis-select your country. Then click 'Confirm Entered Data'.
C2) Look over your information. There's a grey 'Edit' button you'll want to use if anything's even slightly off. If it all checks out, click 'Next'.
D) PAYMENT! This is where things get annoying. You have two payment options: Click-and-Buy, or Crysta. If you choose the wrong one by mistake, go back to step A1.
D1) If you purchase it with Crysta, it's 999 Crysta. If you don't have enough, you're going to have to go buy some Crysta and then go back to step A1.
Alright I can't give step-by-step beyond that point as I'm not buying an actual Security Token or Crysta right now and I'm not touching ClickandBuy. But if you get to this step it's just a matter of giving SE your money and you should be set, aside from waiting for the damn thing to arrive through the mail.
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Once you receive your Security Token, go back to the account management login, and select "One Time Password" again. This time, select "Security Token Usage Settings" instead of "Purchase a Square Enix Security Token", then select "Security Token Registration". Agree to the terms of use again. Punch in your security token's Serial Number (the webpage shows where to find that) and punch in your security token's current one-time password and quickly hit Next before it changes.
(Again, can't give directions past this point, but it should be easy enough.)
Of course you will then need to edit your PlayOnline login to include the One-Time Password setting. Log in and *BAM!* instant extra storage.
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After you confirm you have your storage you can remove the security token... but if you do and your account gets hacked, you have only yourself to blame, no yelling at SE because you removed the security token your own damn self.
Of course, using the security token is a constant-though-minor hassle, so it's up to you to decide where you stand on the security-versus-convenience spectrum[/SIZE].
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Edit:
As a matter of principle, I agree with you that nothing purchased outside of the game should provide meaningful (non-cosmetic) in-game benefits. However, while I disagree with SE's decision to make a form of item storage available only through purchase of a security token, I understand
why they made the decision they did - and I can see that their decision is not unreasonable, especially considering SE may not hold the same principles that I do.
ALso... only your mule needs the extra inventory? Do you have a security token on your main character that is not applying to your mule, or simply don't need the space on your main character?