I would be interested in your comments on the following sentence:
Thank you for your assistance in this matter and we take this opportunity to send you our best wishes.
I can't explain why but it just sounds wrong to me. I think it should either be:
We thank you for your assistance in this matter and take this opportunity to send you our best wishes.
Or:
Thank you for your assistance in this matter. We take this opportunity to send you our best wishes.