Purchase diamonds on Design Home game app and apple keeps over charging 🤬

How come when I purchase something for like $4.99 it charges me $7.99. When I Purchase something for $9.99 each charge me $14.99 I don’t understand with the concept of that is because whatever the app says that I have to pay for That’s what I should be charge for. But I’m charging me more than what they should. Can somebody clarify that for me please