(1)
ما قصرت maа gaSSart (maа gaSSarti if said to a woman) – it basically means something to the effect of “you didn’t come short”, or “you didn’t cut corners”, “didn’t economize efforts”, “you did what it takes”.
Know how to say thank you like a Gulf Arab? No, it’s not just shukran
(2)
الله يعطيك العافية
allaah y^aTiik al-^aafiya
(if said to a woman, it's allaah y^aTiich al-^aafiya ) – this means “May God give you wellness and health”.

It's sometimes shortened to just
يعطيك العافية y^aTiik al-^aafiya
(y^aTiich al-^aafiya if said to a woman)
You can actually use both expressions together:
ما قصرت يعطيك العافية
ma gaSSart, ya^Tiik al-^aafiya
ma gaSSart, ya^Tiik al-^aafiya
or, if you want to thank more profusely:
شكراً ما قصرت يعطيك العافية
shukran, ma gaSSart, ya^Tiik al-^aafiya
shukran, ma gaSSart, ya^Tiik al-^aafiya
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