Hi, welcome to the forum.
On the cover...If you are storing the TT in a shed then why would you want to spend the cash on a cover. For me I wouldn't do it. We are constantly in and out of our TT for one reason or another all year so even if the cover had a zippered door access it would just be a pain to get in and out of. We do store ours inside and without a cover just gets a little dust on it. Gotta wash it for spring anyway.
Of course this just suits us and gives you another perspective to look at. Why don't you wait a year on the cover to see how it goes. It will save you some cash and its covered anyway.
On the warranty...We bought one for some stupid reason...I guess for a little added security. We have never used it but I will if need be. It is not a refundable policy as posted earlier. One thing I did check before purchasing our policy is when it becomes effective. In other words, most of the things on or in the TT are covered by one warranty or another for a certain period of time anyway so our policy picks up when the other warranties expire. So lets say the Dometic refridgerator is warranted for 3 years then my extened warranty policy does start on the refridgerator until the three years are up on it. So I should be covered on it for 8 years.