I really wish I had seen this before I collected 500 survey responses about our ordering system WITHOUT being able to connect them to an order number. It turns out you can pass a custom value into the URL of a SurveyMonkey survey:
SurveyMonkey Help Center – Answers
To customize them, create a unique ID ending for each respondent by adding “&c=” at the end of the link followed by the ID number.
EX: &c=00001
http://www.surveymonkey.com/s.aspx?sm=v8MbvURxoHkWfvud7Or3Cg_3d_3d&c=00001
