Hopper help states that for low cost carriers like Frontier they are using the “merchant model” (ie, injecting a third party payment processor between you and the airline): https://help.hopper.com/en_us/low-co...unds-BkrNUYOtw

I haven’t tried booking a flight yet - if you book Delta or United does the charge still come from Hopper/CapOneTravel or from the airline directly? (“agency model”)