Do you want the PCR test so you can do the test 72 hours in advance, vs. 48??
You can circumvent this issue by using a home proctored antigen test. This type of test needs to be taken 48 hours in advance, and requires an internet connection. If this works for you it will be easier and faster.
When I contacted Silversea they sent me a link for acceptable antigen tests
https://store.optum.com/silversea-covid-19-tests/?utm_source=silversea-at-sea&utm_medium=referral&utm_content=&utm_campaign=optum-store-covid-tests-2022
If you NEED to do the PCR tests you should contact Silversea by phone or email for details.