What happened

Shares of postage payment facilitator Stamps.com (NASDAQ:STMP) dropped as much as 11% in early trading Monday, before reviving to book "only" a 7.1% loss as of 12:15 p.m. EST. Stamps.com hasn't reported anything noteworthy today, nor are its earnings out just yet.

So what

Instead, I think you can blame Amazon.com (NASDAQ:AMZN) for this one.

Three months ago, Stamps.com reported a big earnings beat, but said Amazon was bringing some of its "Fulfillment by Amazon" postage billing in-house and that might affect Stamps.com's business going forward. "Fulfillment by Amazon [is] just another one of the carriers that we support," said Stamps.com CEO Kenneth McBride, and so presumably Amazon's move wouldn't doom Stamps.com's business.

A rhinoceros grazing on grass and standing in the crosshairs of a telescope.

Amazon targets companies with big margins -- and Stamps.com may have gotten caught in the crossfire. Image source: Getty Images.

But investors were not reassured -- and sold off Stamps.com stock.

Today, I think Stamps.com's latest sell-off can likewise be traced back to Amazon and its announcement last week that it's launching a "Shipping With Amazon" service to deliver packages for businesses that sell goods on its website -- and eventually for businesses that don't even sell on its website. That news torpedoed the stocks of shipping companies FedEx (NYSE:FDX) and UPS (NYSE:UPS). It only makes sense that it would affect Stamps.com's stock as well.

Now what

Is Amazon trying to drive out of business every other company involved in shipping packages from Point A to Point B? Maybe, maybe not. But one thing's for sure: Amazon.com CEO Jeff Bezos is famous for his mantra:"Your margin is my opportunity."

That makes Stamps.com, which sports a 34.5% operating profit margin, look like a big fat opportunity for Bezos.