I remember when I had a feature phone & was on prepaid... Telecom had to make the plans weird amounts: For example, $12, $18, $6, $21 etc purely because they knew the top ups only went up in tens $10, $20, $40 etc so you'd have to buy a $20 top up for a $12 plan, buy a $40 top up for a $21 plan... And you might think that you could just use the leftover money next time, but they take away the money if you don't spend it quickly 🙁
Them phone companies... All the same 🙁