I've used this a few times, and there is a way to do it that is much more favorable to the customer.
Let's say your billing date is the 1st of the month.
If, during the month, you realize that you are over your limit, you should wait until the last day of the month (i.e. the 30th) to purchase your transfer package. You will only be charged 1/30th of the monthly cost of the transfer package for that month, even if you use the full amount of download capacity.
Then you need to wait the 30 days before you cancel the package, so you cancel it on the 30th of the next month. You pay the full amount for that month.
You end up paying 1.033 times the transfer package cost, but you get 2.0 times the download capacity!
Just be extra careful not to forget to buy and cancel the package before your billing date!
You can also buy and cancel transfer packages during an online chat with customer service, you don't really need to call.