There is an option with Shippo to only process a charge once the prepaid label has been scanned. This is the so-called scan-based return label.
Shippo currently supports this for USPS, FedEx, and UPS. You can enable this in your Rich Returns dashboard when configuring your Shippo connection and enabling "Scan Based Labels."
For more information on return labels from Shippo and helpful insights, you can check this link: https://goshippo.com/shipping/return-shipping-labels/.
Note that EasyPost does support a variety of return labels as well, which are typically scan-based.
EasyPost now supports Scan-Based labels for USPS. They have confirmed this recently, with more info available here: https://www.easypost.com/usps-returns.
Note that this Return-Rate is different than the usual USPS account and needs to be requested from EasyPost directly to have this enabled on your account.
ShipStation does not support return labels through their API, however.