The rule "get every 5th of its kind for free" applies exclusively to the purchases that have the same price in the Buy Menu and have been made on the same platform (Google Play, iOS, Facebook, etc.). For example, if you purchase one $9.99 item on your iPhone and another one through a web browser on Facebook, they will have separate progress.

Note that when a 2000%, 3000%, or 5000% sale commences, prices in the Buy Menu remain the same, but items available for sale change. It means that if you purchase one item before the sale and a second one during the sale, they will be considered of a different rank. As a result, you will see different progress bars for those items.