There are two ways to allow customers to upload files.
1. Allow him FTP access. You will need to create a folder for the uploaded files, and also create a ftp account, limited to that folder (else he will gain access to all of your site). However, a bad idea if you don't personally know your clients as they could upload malicious code and take over completely your site.
2. Create a form, with one (or more) upload fields. Use
Advanced BlueVoda Form Processor to process the form. ABVFP will upload the files in the specified folder, and will also send you an email with the files attached. You can also make the upload older have some strange name instead of the usual "uploads" so that no one will know the location where the files have been uploaded.