NOTE: this discount method is largely no longer available — Amazon Coins have been discontinued and the Aptoide (AppCoins) workaround is basically unusable now (only some old installs still work). When it worked: buying Amazon Coins in bulk gave a discount (e.g., as of 4/2023, 50,000 coins cost $450 instead of $500 = 10% off; smaller amounts gave 3-8% off), then those coins paid for in-game purchases inside the Amazon Appstore version of Evony (not the regular iOS/Android app). Setup required installing 'Amazon Appstore' (Android only, or via an Android emulator like BlueStacks on PC/Mac for iOS players), installing Evony from it, and switching to your existing account via a previously-linked Facebook or Google account. Remaining live tip: for players billed in non-USD currency, per-platform (iOS vs Android) pricing still differs and which is cheaper varies seasonally, so compare both before buying.
Original research source: Evony Guide Wiki