7zip batch file does not include extension Why?

I found the below batch file and it works fine but I need the code to add the extension to the final zip file. I am sure this is a simple question but I have tried everything.

@echo off

cd /d %~dp0

rem 7z.exe path

set sevenzip=

if "%sevenzip%"=="" if exist "%ProgramFiles(x86)%7-zip7z.exe" set sevenzip=%ProgramFiles(x86)%7-zip7z.exe

if "%sevenzip%"=="" if exist "%ProgramFiles%7-zip7z.exe" set sevenzip=%ProgramFiles%7-zip7z.exe

if "%sevenzip%"=="" echo 7-zip not found&pause&exit

echo %~dp1

echo %~x1

for %%a in (*%extension%) do "%sevenzip%" a "%%~na.zip" "%%a"

pause