On United we were about to complete most of the check-in process online, just not print boarding passes. When we got to the Newark airport SXM travelers were directed to a separate check-in area. An agent at the beginning of the line checked our test results and the pre-approval letter before allowing us to approach the check-in counter were we got our luggage tagged an boarding passes issued. It was a Sunday morning about 6:30am and there were not many people so it went quickly.