In the store you may notice some small red banners on some of the packages that show a percentage. This is not a bonus of rubies that you will receive for your purchase but instead an indication of the savings made for purchasing a package of that size compared to the smallest package available.
In the above example the 15,000 ruby package shows a percentage of 19%. This does not mean you will receive 19% extra to what is displayed, but instead that purchasing the larger package will save you 19% in comparison to purchasing the smallest package 6 times.
The store will always show the amount of rubies you will receive for your purchase, even during a Prime Time ruby bonus offer. This means you will receive what is shown in the store and do not need to calculate your extra ruby bonus.