Transaction Charge - how are transactions counted?

The transaction (txn) charge includes, all unmatched, matched, cancelled and lapsed bets. 

Successful bet cancellations aren't counted as additional transactions.  If you create a single bet (betId), and then cancel the same bet, this is counted as a single transaction e.g. bet placement (1 txn) + cancellation (0 txn), its not double counted.

If you place bet and the attempt results in an failed cancellation, the failed cancellation request will be counted as a failed transaction e.g. bet placement (1 txn) + failed cancellation (1 failed txn).

Please see https://www.betfair.com/aboutUs/Betfair.Charges/ for further information.