Hello Ilaria,
Shipping settings can be changed at any time by saving it on plugin settings page.
In our Demo site, you cannot change and save any fields. It is just to view.
We did not get when you say “Erase previous shipment settings”.
and Yes, Stamps.com Support for US and Canada.
Please raise a request HERE by explaining your exact problem.
We will help you to fix it.
Regards,
Veeresh