"Optional microtransactions" refers to small purchases within a game or app that are not necessary to play or progress. You have the choice to buy them or not. Full definition
This is what the mini / PSM market needs to do for Vita — offer free games with optional microtransactions. (blog.us.playstation.com)
Grand Theft Auto V's multiplayer mode GTA Online is free, but Rockstar offers optional microtransactions for players who want to speed up their progress. (zhugeex.com)
Usually, through the use of optional microtransactions of some sort. (coin-drop.com)