Since Telcel's website won't let me reload my SIM card with US credit cards or PayPal accounts, I've begun to look into online reload services. Recharge.com for example seems to force USD as well as charge a $0.99 service fee; a MX$510 reload costs US$30 before said service fee, or 17 pesos per dollar. (For reference, Google currently says that MX$510 = US$27.05, or 18.85 pesos per dollar.) A brief look at other services seems to indicate that this practice isn't uncommon.

Anyway, while I expect to pay some sort of fee for this service, it seems more fair to me to have the entire fee itemized out instead of trying to hide most of it in the exchange rate.