For anyone who travels via train out of South Station, for parking I recommend you head over to the garage on Farnsworth St (where I used to work), which is about 3 blocks from South Station. Off I-93, take the South Station exit, make a left onto Congress St and follow it across the Boston Tea Party bridge, and then a left onto Farnsworth St. The cap is $12 a day versus $31 at some of the places north of the station – like the Summer St garage.
Also, be aware the station does have wifi – 6 bucks an hour, I think, or 7 for a day. Not great, but could be worse.