Sorry to hear that. Mail passes through so many steps that it does happen occasionally. The local Postmaster isn't your best contact person. Try
https://postalinspectors.uspis.gov/ and register a complaint. With todays technology, mail can be tracked through every step of the process. Most mail is processed through automated equipment, which can unfortunately cause damage. Some problems occur if the package opens and the contents can't be matched with the container. Entry and exit from that system are really the only points of human contact.
I still think the employees we have are some of the best in the industry, but there are bad apples in every group. Gift cards in holiday envelopes are an easily identified target and during the holidays we hire thousands of seasonal workers (casuals) that aren't screened to the same extent as career employees. Supervisors and Postmasters have also been convicted of theft. My only personal experience with theft (and I mail a lot) was a gift card sent to our granddaughter that was sliced from the envelope.
I worked on a lot of cases with the Postal Inspection Service and those folks will take it seriously. Every square inch of larger postal facilities is recorded 24/7. Complaints (especially when a pattern emerges) will get quick attention. Theft of mail is one offense that leads to immediate termination, loss of retirement benefits and usually federal prosecution.
I've seen arbitrators and MSPB judges return a lot of employees to duty after termination, but NEVER have I seen a thief get their job back.