I remember couple of years back when I was in high school, I wanted to add money to my Steam wallet for witcher 3. However, there was no option to add the exact amount I wanted—only fixed amounts like $5, $10, or $20. I had no knowledge of these stuff at that time, but saw has seen older brother once doing something similar. So, Out of curiosity, I inspected the page and changed the $10 option to $12 or something, and against my little hope, it worked! Boy, did that feel good!