"Offer an opinion" or "give an opinion"

While give would be quite acceptable, between the two words, I would choose offer. Not only is it a good solid businesslike word, it is also slightly less demanding, and makes you seem less importunate and more polite.

May I suggest express?

I would like to express my opinion...

Agreed with Daniel (+1) - The difference between the two is offer is more polite insofar as it implies the recipient's right to refuse it, whereas give implies your insistence that the recipient accept your opinion. If the rest of your response is very strongly worded, then give would be an appropriate choice as it will match the tone of your strongly held opinions.