USPS ShipNClick, for example, has "fraud detection" that freezes all accounts you've ever logged into if it detects that you've created multiple accounts and switched between them. And worse, it's a silent detection, so after you've entered all the details for your shipping label(s), it gives a "Your payment method was declined" error. No warnings or anything, just a vague error message.
When you call USPS online support, they're aware of the issue but they can't unfreeze accounts without "escalating" the issue. You get a callback in a few days saying that your account was "cleared".
You can see a small gif of it working under "How you make money" in the interview. https://cdn-images-1.medium.com/max/800/1*GDC26ygGdl49WKsb9Z...
edit: if anyone else would like to chat, sales@goshippo.com is the best place to get to us. That goes directly to a few of our inboxes, so we're get back to you in no time.
This is strictly for one-time shipments, targeted at the occasional sender. There are better solutions out there for small businesses.