I am typically using it to indicate that specific portion of textual information is different or less important (auxilliary) than the rest:
Order Rejected (No money left)
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Italics doesn't mean anything in and of itself. Context is what gives it meaning. In my first sentence, it means emphasis.
But it can just as much mean de-emphasis in other contexts (such as your example)
Ultimately italics is just a way to differentiate text. Use it as it makes sense to in the context of your UI.
First of all, I'd say your example shows incorrect use of italics.
Understand how traditional typography guides the use of italics: http://desktoppub.about.com/cs/finetypography/ht/italic_type.htm, http://practicaltypography.com/bold-or-italic.html. Web typography might deviate from traditional print typography but not by much.
Italics add subtle emphasis. Indicating a proper noun is one use.
There are many claims that italics are less legible, especially for the visually impaired, so italic text might be more difficult to read. (This legibility issue is difficult to quantify as it depends on the typeface, whether the typeface has an intelligently design italic variant [as opposed to a simple slanting transformation of the regular font], and the medium [resolution, contrast, etc.].)
The typeface can also influence the use of italics: italics usually look better with serif fonts and are less attractive with sans serif fonts, so some prefer to use bold to add emphasis using sans serif fonts.
I have intentionally overemphasized in this post to make a point: take care not to overemphasize.
Depending on the font, italicised text is less legible.
Wouldn't it be better to phrase the information so that it is all of a piece?
e.g. "You need money in your account to order goods." (Too long, but you get the idea)
And then add a call to action button - "Top up funds" (you don't want to lose the sale for lack of a CTA).
If you wanted to stick with your original wording, I would format the supplementary information as smaller, paler text, e.g. grey, or dark grey on a grey background. But be careful to have sufficient contrast.
Short answer: never use italics on GUI elements.
One important reason is that italic type only applies well to roman alphabets, so if you ever want to localize your interfaces to languages with non-roman alphabets, you won't be able to rely on italics.
Do use them when needed on text blocks of content, for which follow the language's grammar rules accordingly.