Returning from Puerto Rico: no customs process at all — it's a US territory, same as flying home from any other state.
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No — Puerto Rico is US territory, so returning to the mainland is treated like a domestic flight with no customs process.
There's no customs exemption to worry about at all, since no duty applies to begin with when flying between US territory and the mainland.
Puerto Rico is US territory — there's no customs process at all when you fly back to the mainland.
Yes — you'll still need a valid government-issued photo ID for the TSA checkpoint, exactly as you would for any other domestic flight.