I don’t believe they overcharge, but a lot depends on you and how you enter what you want. The maximum amount via the phone in which the system will not accept is $1,000. I entered that accidentally thinking I was entering $10.
The automated system gives the increments below:
$29.99
$49.99
$69.99
$99.99
option 5 lets you enter the amount you want. If you select option 1, the call will bill you at the per min rate which is perhaps almost always going to be more.