By default, ShipEngine provides hidden postage (hides postage paid) on USPS labels. However, if carrier insurance is added to a USPS shipment or if the shipment is First Class Mail and package type "letter", the postage paid amount must be displayed on the label regardless of the postage provider (Express 1 or Stamps.com). Therefore, if you want to keep using stealth postage, add Shipsurance to your package instead of carrier insurance.
Express 1 has NSA (Negotiated Service Agreement) rates, so they are prohibited from displaying their rates on labels. However, this rule is overridden by the USPS requirement to have postage shown on all labels that have carrier insurance added to them. Additionally, some countries (like Brazil) require postage to be displayed.
For more information, check out the Shipsurance blog article on the subject.