I'm not terribly familiar with his autograph. I would suggest you do a search of this site for "Global Authentic", the sticker on that helmet. There is hours of reading.
I don't know anybody who has any faith in anything with a GA sticker/COA.
Personally, for Manning I'd look for something with a Steiner COA. Manning signs for them. Everything with a Steiner COA was signed in their presence. The price will certainly be allot higher. But the autograph will be real and if you ever resell the item it will hold its value.