The "baggage service" you paid for has nothing to do with U.S. customs--it will help you avoid Canadian customs. That being said, you will probably pre-clear both US customs & immigration in Vancouver (assuming they still do that there, but I don't see why anything would have changed) and then your flight is essentially a "domestic" flight when you get into Detroit and you won't have to go through any of the re-checking, etc, nor will you have to go through customs again.