What's the catch with Stamps.com?

I just signed up for an account with stamps.com. I got $5 free postage and a free postage meter. There are no monthly fees. I just pay for the postage I print. And the postage I purchase from them is sold at the "commercial rate" (you can see the difference at all rates at https://www.usps.com/business/prices.htm)

For example, a first class stamp is only .48 rather than .49, and a Priority flat rate envelop is $5.05 instead of $5.75. I can print first class mail on Stamps.com "net stamps" labels (they cost less than a penny each) or directly onto an envelope in my laser printer. If I print a whole sheet of laser net stamps they are good forever just like a sheet of stamps purchased at the post office.

As of January 25, 2015 the "net stamps" can only be used for first class mail - they can't be used for packages or priority mail (because the post office requires a tracking bar code and the net stamps are not big enough to include one.) However, postage for priority mail, packages, and other non-first class mail can be printed on any correctly sized label or even on a plain sheet of paper then taped to the package.

So what's the catch? I can't find one. Except I have to buy a "next window please" sign to put on my laser printer now and then so I can replicate the post office experience at home.

What makes you think there is a catch?

I have many clients that use paypal to purchase shipping for the post office and it works rather well. You can purchase shipping including tracking. You drop your packages off at the post office and they just scan the tracking bar code. Done. Pretty simple.