In your example, I am not sure the "it" refers to. I guess it is the team? You could say any of the following. In each case appropriate could be a substitute for suitable. in a suitable manner in a manner you consider to be suitable in a manner the team considers to be suitable All of these are polite. To make the overall sentence even more polite you could say "if your team would kindly convey this .." or "if you team would please convey this ..." All of this are correct and polite. I hope that is helpful.

No, your sentence does not say what you want it to say. Your sentence says that you want the delivery method, not the letter, to be appropriate. For example, it might be appropriate to deliver the letter in person, but inappropriate to fold the letter into a paper airplane and fly it through Mr. A's window.

Generally eastern people overdo with politeness when deal with westerners who live happily without two bows for each breath.