I'm interested in purchasing a Sunjoy product and have noticed that this product is available at my local big box stores as well as Amazon for a significantly less price than I'm seeing on your website. Are these product at the big box stores the same as on your website or are they different and if so how? The item that I'm looking to purchase is a 9 X 12 Black Steel Pergola with Beige canopy. I'm wondering if this canopy can be purchased separately at a later date if needed. Thanks you for your response.