Hi Michelle
I've used MyUs quite a few times now, and everything is completely legitimate.
You can choose between three membership types + you pay a one-time set up fee. These are the rates for Canada:
http://myus2.myus.com/Memberships.aspx
You can order from most American on-line stores, at check-out you then enter your 'American address' and all the goods are shipped to that address (Florida). They can store everything there for you up until three months (for free), after that you pay a fee. You decide when everything is shipped to you, and then the following rates apply:
http://myus2.myus.com/Rates.aspx
Disadvantage: if the online store does not offer free shipping, you pay double shipping fees.
Also: some countries charge hefty custom fees (like Belgium), so that's something to take into consideration*. (Tip: sometimes it helps to check the box 'Declare merchandise at wholesale value')
Oh yes: Some online shops don't allow you to buy things with a foreign credit card (e.g. Bluefly). If you like you can then use the MyUs Personal Shopper Service: you enter what you'd like to buy via their site, and they buy it for you (and charge the items to your credit card). You pay $10 dollars for this service.
* I've been very lucky to have had lovely lasses like Patience and Maya help me with ordering items in the US (thank you again girls!!). It did keep down the costs. But just so you know MyUs is definitely a reliable option.