A lot easier...
I did it in 2 minutes.

Dim oFso As Scripting.FileSystemObject
Dim oStream As Scripting.TextStream
Set oFso = New Scripting.FileSystemObject
Set oStream = oFso.OpenTextFile(FILE, ForAppending, True)

oStream.WriteLine tPosting.sCustomerCode & (...)
Set oFso = Nothing
Set oStream = Nothing

It's just that when I have something that's going wrong, I want to know why.
I didn't find out this time...