What is the difference between “obrigado” and “obrigadinho”?
It depends on your tone Grammatically speaking, the only difference is that “Obrigadinho” or “Obrigadinha” is in the diminutive, which we often use for emphasis or to be extra affectionate, for example. So, you can say “Obrigadinho” or “Obrigadinha” simply to emphasize how thankful you feel or to say thanks in an extra nice way. But we often also use it very ironically or even snarkily, like when something doesn’t go the way we intended.
Filho: Pai, posso ir à festa? (Dad, can I go to the party?)
Pai: Não! (No!)
Filho: Obrigadinho, 'tá?! (Well, thank you very much, 'kay?!)