Unfortunately, and sadly, gift card theft and pilfering is rampant.
OP - I'm so sorry that this has happened to you. I have been in your shoes and would rather go barefoot than deal with that chaos again.
I've had about 5 instances of gift cards being stolen through the USPS. I've also had two instances of gift cards being pilfered while on their way to me. I received the cards, but either they had already been used entirely or there was an amount missing from them and the entire original amount ($25, $50, $100) was no longer available.
I will never again purchase Visa Vanilla cards because they will not refund you a new card or take any measures to reimburse you if you're a victim of gift card fraud/theft. Imagine your child getting a $50 gift card from a relative for Christmas and the money already being gone from the gift card because hackers are plugging in random numbers and !viola! they get lucky and steal your child's gift card money before it even arrives in the mailbox! Yes, yes, one very sad child at Toys R Us when the gift card doesn't work. And very confusing and frustrating for the adults who have to deal with it.
It's my experience that some USPS workers open Christmas cards, birthday cards, or generally any cards that they think may contain gift cards. Out goes your gift card into their pocket and nobody is any the wiser, unfortunately. I can't tell you how many times I've gotten mail that was opened...
There's been a sting recently where companies mail out gift cards and track those numbers and are working in conjunction with shipping companies (and I believe the USPS) to track them and catch the insider theft.
For what it's worth, I only use my credit card to make online purchases now (and I never purchase gift cards online anymore, I do e-gift cards). My credit card company will fight the good fight for me -- as they should!