I thought that you were not allowed to say thank you for meshlo’ach manot. Is that true?
I don’t think this is true. The reason we give shalach manos is to enhance friendship among Klal Yisroel, and if we are not going to reciprocate and say thank you, firstly we are showing that we are not grateful for the present that the other person gave us, and secondly it will not foster friendships, but the opposite. Therefore we should be very friendly and give the person that gives us shalach manos a big hearty thank you.
It is possible that you might be mixing up getting a loan from someone, that since there is a prohibition not to give the person interest, there where there are opinions that we shouldn’t thank the person afterwards. Many poskim though say that while we should not bless the person, but we may say thank you.
Best Wishes and a happy Purim