Done it several times with gold coins. It sounds like you are asking about entering the USA on an international flight - you won't be interacting with TSA on arrival, you would be dealing with CBP. You need to declare more than $10,000 USD in currency from memory. I had a big delay once carrying US pre 1933 gold coins in - until another agent popped in and told the agent dealing with me "hey, they're US currency, there's no duty on them". Filed a FinCEN report and was on my way.
HOWEVER - to be considered currency by FinCEN, where the report goes, the items have to be intended or have been in circulation - so bullion Pandas, while legal tender, do not necessarily meet the definition of currency for reporting purposes.
https://www.fincen.gov/resources/st...s/fincen-form-105-currency-and-other-monetary
Pandas might be considered currency on entry - but most regulations explicitly mention gold, not silver or precious metals. It would be hard to carry $10,000 in silver internationally anyway.
https://help.cbp.gov/app/answers/detail/a_id/322
https://help.cbp.gov/app/answers/detail/a_id/195
Safest to bring a receipt, invoice etc that shows the value and proof of purchase. Try to avoid having an agent decide that they are personal effects, as then you may be forced to pay import duty on them on the spot. If you bought them within the last 12 months though then if they get declared personal effects then you'll almost certainly be in a position where an agent will be deciding if you're going to be paying duty on them. If you have receipts showing that they were purchased more than a year ago bring those. Almost guaranteed that the agent won't know how to handle it.
The quantity will affect how it goes - if you're bringing in half a dozen, then they're basically souvenirs. If you're bringing in a hundred things will get tricky and they make you use an import agent.
You can always try calling US Customers and Border Patrol. If your're a frequent traveller to the US get Global Entry status - makes things a *lot* easier on arrival.
