NO AMAZON REFERRAL FEE

hi Everyone,

I have noticed that I have not been charged any referral fee since the beginning of the month until today. I am Kind of not so new seller as I have been selling for past 4 months. Not sure how many items are free from amazon referral fee each month. I have counted that around 20 items i have not been charged. I have searched amazon but it does not state anyting about referral fee discount.

Thanks