As of now, there is no copy command in RDS Postgresql but there sure is a way to do it on our own.
Here's how you can do it:
1. Install AWS CLI on your EC2 box (it could also be installed by default)
2. Configure the AWS CLI with credentials
3. Download s3 to your local directory using aws s3 sync command as shown below:
aws s3 sync s3://bucket/file.csv /mydirectory/file.csv
4. Use the following command to copy the files into your RDS:
psql -h your_rds.amazonaws.com -U username -d dbname -c '\COPY table FROM ''file.csv'' CSV HEADER'