Sorry you feel you were somehow scammed by Mapache. I think scam is perhaps a strong word.
I assume that when you say bag tax - you may be referring to the airport departure tax.
Are you saying you purchased that tax from Mapache and they charged you $32? Did you then have to pay the amount of the tax again at the airport at the tax counter? Or are you saying that the $34 you were quoted then turned into $577-538 = $39.
The truth about the $28 tax is that it is $28 that gets charged to you by the Airport Authority. Thats what they get. However if you pay it with a credit card, the transaction is treated as a cash advance and the credit card company adds an extra charge. Most people who have done this and reported back to the forum say that it was $10 that was added to the transaction as a cash advance fee. (The amount may vary depending on the CC ) So I think this is where the $38 for a credit card payment probably comes from. The Mapache employee was actually not wrong. $38 is what a person would end up paying if they used a CC at the airport. I assume you wereprepared to pay in advance with a credit card for convenience of skipping a line at the airport.