Why does Amazon charge you 0.00 fulfillment fee for some orders when you change the price of your product to $9.99 or less?

Good morning everybody

I have done some tests and changed the price of my product (PL) from $29.97 to $9.99 in order to clear stock. So I came across some kind of promotion when the price is $9.99 or less. Amazon did not charge for shipping, only the fee (8%). Below is Asin's report on my product with the latest sales, see that some do not charge for shipping and others do.

The question is :

How does this happen?

Are you part of a program?

Is it possible for Amazon to waive the

for my product when the person buys a different item, so it gives the discount, as the other product has already paid for shipping?

I remember that in the first few days that I changed the price to $9.99, all sales were made at only 8% Fee. After a week it returned to the normal fulfillment fee price (8%+ Fulfillment Fee)

Does anyone know how to explain?