Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of Stamps.com (NASDAQ:STMP) have popped 13% today after the company smashed through earnings expectations and provided positive guidance for the rest of the year.

So what: Stamps.com's analyst crew had been looking for revenue of $31.4 million and earnings of $0.47 per share. These were a bit on the low side, as Stamps.com came in with revenue of $32.1 million (up 14% year over year) and $0.60 in EPS. With half the year over, Stamps.com is now offering full-year guidance that anticipates a revenue range of $125 million to $135 million, and an EPS range of $2.00 to $2.20. Revenue projections are nothing to write home about -- the Street had modeled $129.1 million there -- but EPS expectations are well ahead of the analyst consensus of $2.04.

Now what: This is the second big pop for Stamps.com this year, and its stock is now approaching a double for 2013 (it's already more than doubled over the past 52 weeks). It appears that all of this growth has been on the back of valuation expansion, as Stamps.com's P/E has risen 180% in the past year. That P/E is now in line with historical norms, which indicates less potential growth ahead than investors might have found at the start of this year, when Stamps.com's valuation was dawdling at multiyear lows. It might be time to hang back for a while with this one.

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Fool contributor Alex Planes has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.