In order to conclude a letter I'm writing in Spanish for services offered, I'd like to say "it has been my sincere pleasure assisting you with this matter". Could anyone offer suggestions for the best way to say this in Spanish? Thank you!
I agree with DCPaco and Montevideana about the "sincere" matter.
I'd only change "con" with "en" as it is more proper in spanish.
I would also omit the ", para mí," part unless you want especifically make clear it is you, personally, and not anyone else, who is pleasant with the matter. Without it, the letter will be more formal.
It would be: Ha sido un placer ayudarle en este asunto.
If you want to include the "para mí" part, I would write: Ha sido un placer para mí ayudarle en este asunto or even Para mí, ha sido un placer ayudarle en este asunto, but in the last sentence you seem to imply there was something really personal around.