For each tax year, you file and pay separately. You do this whether or not you file an amended return, and regardless of what is on the returns themselves. It sounds like, if I understand your question correctly, you are now going to file an amended return? In any case...
No, don't attach the letter. First of all, you're not supposed to attach such a letter... you could if you wanted to I guess, but that just gives the IRS ammo. Second, you are to retain such documentation in the case of an audit, and during an audit, I highly suggest you supply only the information requested. Finally, if you deducted the service dog on your original return, and you are not changing anything with respect to that deduction, then attaching a letter about the service dog to your amended return is just going to get confusing and make it more likely you'll get audited.
Side note here, sorry: Your question is hard to understand because you used very poor grammar and run-on sentences, so if I'm not understanding you correctly, reply to let me know and be clear when you do. This helps me answer your question correctly.
Thank you for your question.