I wrote a prayer for someone who passed-away to someone in Greece who speaks French. They answered back Merci Robert. So how do I respond since there are so many ways to respond, what is the correct way to say your welcome in that instance? They seem to use a very informal dialect. Would de rien be correct? I do not know this person, by the way.
"De rien" might seem a bit flippant for this kind of occasion.
You could use "Il n'y a pas de quoi," "Il n'y a pas de quoi me remercier," or come up with something less formulaic that expresses your actual thoughts on the matter. For example, you could say "C'était normal que je le fasse," "J'étais content de pouvoir le faire," or something similar.