As with any warranty/service contract I would heavily weigh the value of what you get. In this case you are receiving an additional 3 years of coverage for $90.00.
On Google it shows that the unit itself costs approximately $500. $90 for a contract on a $500 purchase seems a little excessive representing 20% of the purchase price.
I used to sell vehicle service contracts - a real value would be in the 3.5-5% range. I used to have quite a few 7 year factory extensions to the 3 year basic that only cost a customer around $1300.00 on a $50,000 vehicle.
A lot of the time the service contracts are a big money maker for the company selling them with huge margins - but not always. Honestly I wouldn't pay anymore than $25 for that coverage on that unit however that is most unlikely if it's a fixed,take it or leave it, price.