Signature confirmation is available through Shippo for all carriers, whenever the carriers themselves offer it.
USPS only offers signature confirmation for Priority Mail, Priority Mail Express, and First Class domestic packages. It is not available for USPS international shipments - it is only offered for domestic shipping. Note: There is an added cost of $2.60 for USPS Signature Confirmation. You can read more about USPS Extra Services here: https://www.usps.com/ship/insurance-extra-services.htm
UPS, FedEx, and DHL Express offer it for all services, both domestic and international.
If you see the error message at the bottom of the rates page that says “Error calculating postage rates for one or more mail classes - selected services do not apply,” that indicates that shipping confirmation is not available via USPS for an international shipment.
To add signature confirmation, click the “Add signature confirmation” checkbox on the page where you enter package dimensions and weight.