Hi Kathy,
I haven't actually tried this in PI before however there is no reason why it shouldn't work.
Produce your CSV content from an XSLT mapping with the output format specified as text. Pass this to the receiver plain HTTP adapter and in the communication channel specify the content type so that it matches what the website expects in the HTTP request. This might simply be text/plain.
Here's a link to the receiver HTTP adapter documentation:
http://help.sap.com/saphelp_nw04/helpdata/en/43/64dbb0af9f30b4e10000000a11466f/content.htm
Hope this helps!