I am not sure what free credits you get when using credits to make a purchase but I do know that getting credits you are due is akin to being on hold with your internet provider or IRS (which I have experienced - most unfortunately).
I was owed thousands, by my calculation, for surveys that I was disqualified from only after answering many questions to see if I qualified. It appeared to me that by answering the questions in order to see if I was eligible to take the survey was basically taking a survey to see if I could take a survey just to find out that the survey I took resulted in not being qualified to take the survery I wanted to take. So, I started keeping track of dates/times of the surveys I took to see if I was qualified to take the survey I wanted to take by taking the survey to see if I was qualified to take the survey.
After a while, I wrote to WGT member services and demanded over 3,000 credits for the surveys I took, to see if I was qualified to take the survey I wanted to take, just to find out that the survey I took, to see if I was qualified to take the survey I wanted to take, said I was not elgible to take the survey I wanted to take, after having taken the survey I did take, in order to see if I was qualified to take the survey I wanted to take.
At first, I was being ignored but...WGT did not know me then...So, I kept at it, every day, sending long emails asking for the credits due me for the surveys I took to see if I was qualified to take the survey I wanted to take which, based upon the survery I took, resulted in not being able to take the survey I wanted to take and that was after the taking the survey that I did not want to take in the first place.
After many many many emails, WGT gave me the credits. Afterwards, I received an email from WGT asking how they did and asked if I would take a survey about my experience. As their was no credits involved, I did not take the survey.
I hope this helps