How to turn a technical screw up into a "delighter" feature

I received this recently from Xero.com. Because they reached out honestly, pro-actively and generously I ended up actually feeling happy as a result of their (initial) failure. Kudos.

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Hi Peter,

We have found an error in our billing system which caused your credit card to be charged twice for your monthly subscription to Xero.

This occurred twice on the same day, 30 November 2009.

We’re really sorry about this. We have investigated how this error occurred and are making system changes to ensure that this cannot happen in the future.

We apologise for any inconvenience this may have caused your company.

We want to rectify this error for you as quickly as possible and would like to offer you two choices: 1. We can make an immediate refund of this amount directly to your credit card 2. Or we can apply the overpayment to your next invoice.

Regardless of which option you choose, we’ll also give you one month’s subscription for free as our way of saying ‘Sorry’.

Please let us know which option you would prefer, however if we don’t hear from you we will apply the overpayment to your next invoice.

Once again we apologise for any inconvenience caused.

Kind regards Ben