I believe Delta code shares with Silver but the fact I'd be using Delta miles (some Delta miles mostly Amex points which can transfer to Delta) seems to prevent booking through to EIS. I suspect baggage will have to be claimed and rechecked to Silver in San Juan. We'd also have to get separate tickets for the last leg which can create problems if there are flight cancellations.

These issues are why for the last 2 trips we went to STT and took the ferry. However mileage requirements to go to St. Thomas are about 50% more than to San Juan.