Pre-Paid Cards

[Silumesii Maboshe]

Hello!

I'd like to use Ledger to track things like pre-paid cards, Skype
Credits, reward points and the like. Say I bought 14.00 Skype Credits
and paid $14.00 for them on a MasterCard. How would I record such a
transaction and show that 14.00 Skype Credits translates to $14.00?

2009-03-20 Buy Skype Credit
        Assets:Pre-Paid:Skype Credit            14.00 SkypeCredit
        Liabilities:MasterCard

Or should I add the actual credit value to the credit card like this?
2009-03-20 Buy Skype Credit
        Assets:Pre-Paid:Skype Credit            14.00 SkypeCredit
        Liabilities:MasterCard                          $-14.00

The notes on commodities in Ledger.pdf are useful but I am still
unclear on how to use those.
Thanks in advance.

Silumesii