how do i cancel a pending purchase that i already payed for?

so, i bought robux on the roblox mobile app. i chose the 80 robux option twice, both times went through successfully and it even took money from my card. i didn't choose the subscription which is 4.99. now wll of the suddenly, despitethe purchase being successful- apple is saying its still pending?? now, i deleted the roblox app because of some issues that had nothing to do with this, and now i cant download it again because i have to pay another 2.14 dollars- i already payed it, so why is it charging me it again?? its been doing this for almost a year. i lost so much money because of it and im a very poor person. im low income and this just makes it worse. whatever money i been able to scrap just goes to waste because apple wants me to pay extra?? this is a serious problem that needs to be fixed, please. i cant keep losing money for something i already bought..