First of all Im not that good at critiques, I usually ask others to do so with my writing, but a few things stuck out for me. Note I quickly scanned your letter.
I think you should shorten it, it would make it more powerful if the reader could get the meaning in less time.
Also a lot of the paragraphs start with or end with the word 'volunteering'.
Try to think of synonyms which could be used in exchange.
I feel you are excited about this opportunity. I hope it goes well for you and wish you the best.