Some express shipping services do allow for return labels to be generated, however a few do not.
Below are the shipping services that do not allow for return shipments.
Priority Mail Express:
Return shipments generated in ShippingEasy cannot be sent via USPS Priority Mail Express to ensure that delivery expectations are well managed. Since Priority Mail Express is a guaranteed service, with a postage refund available if the service commitment is not met, it's imperative that the package enter the mail stream on the correct ship date. As the postage purchaser (you) cannot guarantee that the package is delivered to the USPS on time, this precludes USPS from further guaranteeing that the package is delivered back to you on time.
FedEx 2-Day Express