Just signed up with mailjet and I am trying to insert the php files into the site so as to be able to link it to my white label account with mailjet. Has anyone ever done this before?
No,nobody has ever done this before, because it is impossible: Wix does not support php. You will have to develop JS-code against a mailjet api (if there is one, and there prob. is, otherwise there would be no php code) using wix-fetch.
Or … you could use my solution as described here https://girizano.wixsite.com/codecorner/home/send-email-thru-gmail-and-others-in-wix-code using emailjs from Wix Code, because emailjs also supports mailjet (I just checked for you).