Find all files in the current directory (maxdepth 1) that are either .csv or .txt files, search for the string 'string' in them, and then compress them into a file called backup.tar.gz.
find . -maxdepth 1 -type f \( -name "*.csv" -or -name "*.txt" \) -exec grep -il 'string' {} \; | xargs tar -czf backup.tar.gz