I also sometimes postpone thanking a corrector because I want to look at the correction first and maybe ask questions but there are correctors who really get upset if you don't thank them within the day. If they see that you were online but didn't thank you, they might delete it. I think only a few correctors react like this but they do exist. Best is to quickly upvote the correction and just write "thanks" or "thank you very much" to be on the safe side and then you can look at the correction in detail later.
The italki points that you receive for correcting notebook entries, answering questions, posting discussions etc. are only an indicator for your activity. Users who help other a lot tend to have a high number of points. But they don't receive anything in return by italki. You can't turn the points into credit.