[mothered]

the order was shipped correctly as stock numbers match the amount in our system

So what? It doesn't mean "you" got It, It could've been shipped to anyone.

I said the envelope was sealed before I opened it, as I have read that saying it’s tampered with leads to a courier investigation

If you're thinking of opting for the latter (tampering), you cannot change your story. Doing so, Is an admission of guilt.

Request your claim be escalated to a senior representative.