As Shhmokewear just mentioned, it's not true integration. I personally called Stamps.com today and their customer service agent agreed that the information on that page was very misleading (which they will be changing).
You can export from Ecwid, but when trying to import into Stamps.com, we ran into problems today because of:
1. Weird symbols in the output
2. Dropping of the leading zeros in zip codes
Then, when importing into Stamps, there were also some weirdness with address validation.
Not saying that those things can't be overcome, but it still seems like you'd have to either 1) manipulate the exported data in the csv, or 2) get Ecwid to modify the export to help the process along.
Lastly, even if you could streamline the Ecwid -> export to CSV -> import into Stamps.com -> export Tracking from Stamps.com to CSV, there still isn't a "import into Ecwid" function that would update shipping status and place the tracking information where it needs to be.
From our pov, Ecwid should be the system of record, so importing the tracking information is just as important.
(For those who care, we'd love to see at the very least a simple Edit order function and a way to print out multiple invoices - we just had to print out 50+ invoices and had to press "Print Invoice" 50 individual times. Ugh.)