Stamps.com: Dirty scoundrels
I'll get the "Pro" done quick: The performance of the product is great. It does what is says; you can ship via USPS on their somewhat clunky software.
Cons: Dirty, lock-in membership model. They make you buy postage up front in bulk as well as subscribe to a membership monthly, easy and online mind you. When you want to cancel they make you call an automated telephone number to cancel your account. Once you get to the end of a long numbered list to enter in their telephone system you wait because "their customer service agents are all busy." Convenient.
Once talking to the agent, they try to re-sell you multiple times even after I told them, "No more offers."
A month after I thought my last bill would be charged I get another $15.99 charge because they "bill in the rear."
Dirty old school tactics to keep you locked in to your account. Let me signup online but call to cancel and then bill a month late. Gross. Did I mention it takes weeks sometimes to get your "bulk purchased" postage refunded in your account?
TL:DR: Easy online signup but you must call to cancel which is difficult and lengthy, THEN they bill-in-rear the past month after you cancel.