The 'a' command for 7-zip is simply archiving the files under the name specified ("%SSIS_FTPFILE_DEST%\PluSales\PluSales.zip"). Do you want the name of the archive to be created differently? You should be able to change it in the script to whatever you like.
Syntax of the statement:
Program path = "C:\Program Files\7-zip\7z.exe"
archive command= a
archive file name = "%SSIS_FTPFILE_DEST%\PluSales\PluSales.zip"
source file name = "%SSIS_FTPFILE_ORG%\PluSales.csv"
output to command = >>
log file name = "%LOGFILE%"