The charges that sit on top of interest: opening, annual, account maintenance, card replacement, late payment. CONDUSEF (Mexico's financial consumer regulator) keeps a public registry called RECO with everything every bank is allowed to charge. If three mystery charges show up on your statement in a single month, look them up there — if they aren't registered, they're illegal.
The ones that hit hardest in Mexico: opening (1% to 3% of the amount, sometimes financed into the loan), card annual fee (500 to 8,000 pesos), cash advance on a card (up to 8% of the withdrawal plus interest from day one) and late payment (350 to 700 pesos per miss).
All must appear on the cover page and, summed, are already inside the CAT — which is why comparing CAT is more honest than comparing rates. A practical tip: the cash-advance fee on a card is among the most expensive things in the system; if you need cash, a personal loan almost always beats pulling it from the ATM with your card.