Be careful here. Not all B fares are refundable (which is fundamentally idiotic).

When you're booking your trip click the "refundable" button as you're generating your search and that will create a refundable itinerary for you. It might be B, it might not be.

Before you purchase, double-check the fare rules, which will clearly state whether the fare is refundable. And, you always have the 24 hour buyer's remorse recourse if you find out that you accidentally purchased an non-refundable refundable ticket.

No fees should apply for a refundable ticket.