OTAs
 

Any advice on which OTAs recommended by Matrix/ITA PowerTools are reputable? Any other advice on how to buy fares found by ITA Matrix or Google Flights? I'm specifically trying to but the AM BCN-XXX-MEX-GRU fares. But, only a couple will take me through to the purchase page without repricing. And, they don't offer 24 hr. cancellation without penalty.

Thanks. And, if this isn't the right place to ask this question, I apologize.