thank you for contacting me
sorry to bother you and mr. @Arde
they really could mean what you like, arde is always giving good ideas, but i just dislike the usual “up” as it might be a bit pretentious, those letters are humble and could virtually mean anything
Very clever. It forced me to explore Spanish phonology. Most speakers in Spain (except for Western Andalusia and all Canary Islands), have both /θ/ and /s/ (distinción). Therefore I conclude "su" phonetically means "sank iu" which in English would be spelled as "Thank You".