This would be a third-party booking site like Expedia or Orbitz. These sites typically give you a 24-hour cancellation window for all tickets (as required by US law).

Just two notes of caution on this approach:

1. Your name in the booking via a site like Expedia must exactly match the name in your M&S account. I once wasn't able to upgrade an Expedia booking because Expedia included my middle name, whereas my M&S account didn't have the middle name.

2. If you book via Expedia using a non-US credit card, you may lose money during a refund process due to exchange rate fluctuations. I've booked before using a South African credit card and cancelled within a few hours. But due to the refund posting on a different day, I lost about $15 due to exchange rates.