Import-csv Multiple files - powershell

Question

Simple Task. Just trying to loop through a folder of .csv files and import them. I'm a bit confused as this does work on other scripts, but I must be missing something very obvious.
$CSVFiles correctly finds 3 files in the folder.
However $csvOutput only contains data from the last file in the folder.
The rest of the script is working fine.
$CSVFolder = 'D:\XXXX\2021_MAY\'
$OutputFile = 'D:\XXXX\2021_MAY\merged_MAY.csv'
$CSVFiles = Get-ChildItem -Path $CSVFolder -Filter *.csv
foreach ($csv in $CSVFiles)
{$csvOutput = Import-Csv $csv.FullName
$group = $csvOutput | Select-Object
'Channel Reference',
'Received Date',
'Currency',
'Payment Method',
'Channel Payment Reference',
'Line Total Excluding Tax',
'Order Tax','Order Total' | Group-Object {$_.'Channel Reference'}
$group | Where Count -gt 1 | Foreach-Object
{
$_.Group[0].'Line Total Excluding Tax' = ($_.Group | Measure-Object 'Line Total Excluding Tax' -Sum).Sum
}
$group | Foreach-Object { $_.Group[0] } | Export-Csv $OutputFile -NoType
}

Answer 1

I've now fixed this
$CSVFolder = 'D:\XXXX\2021_MAY\'
$OutputFile = 'D:\XXXX\2021_MAY\merged_MAY.csv'
$CSVFiles = Get-ChildItem -Path $CSVFolder -Filter *.csv
foreach ($csv in $CSVFiles){
$group = Import-Csv $csv.FullName | Select-Object 'Channel Reference','Received Date','Currency','Payment Method','Channel Payment Reference','Line Total Excluding Tax','Order Tax','Order Total' | Group-Object {$_.'Channel Reference'} |
Foreach-Object {
if ($_.Count -gt 1){
$_.Group[0].'Line Total Excluding Tax' = ($_.Group | Measure-Object 'Line Total Excluding Tax' -Sum).Sum
}
$_.Group[0]
}
$group | Export-Csv $OutputFile -notype -append
}