Here's how to upload documentation to the Resolution Center:
- Go to the Resolution Center.
- Click Respond on the case row.
- On the case details page, click Respond.
- Choose I'd like to submit the requested info.
- Submit the required information along with a description.
-------------------------------------------------------------------------------------------------------
In most cases you wont need to submit any additional information, but just in case, you should keep a copy of all order and communication to and from the seller.
I don't remember if you can just upload information before it is requested by PayPal, but it should not be needed unless requested.
Hope this helps some and there is a lot more that can be found in the help files with a little digging.