The ‘biggest question’ about the Capital One-Discover deal? It revolves around fees you probably never see.

Capital One Financial Corp.’s possible purchase of Discover Financial Services could prove a costly shake-up for credit-card users, according to consumer advocates and payments experts.   The potential $35 billion merger has prompted concern from consumer groups, who say it could allow Capital One COF, +0.12%to push annual percentage rates on credit cards even higher in an already-high interest-rate environment. Others say the most notable potential change could happen on th