Hi Kirby,
What you have here is a common hurdle to many online businesses. Just like a company selling T-shirts, they may have a smiley face t-shirt in s, m, l, xl, xxl and in 5 different colors. So how does one optimize their content to be found when somebody searches for XL red smiley face t-shirt?
You can take one of two approaches.
Option 1: You can optimize a page for the "main item" in my example it would be "smiley face t-shirt". Then try to get the long tailed keywords on the page by listing colors, sizes, on the page a couple times. The goal of this page would be to rank #1 for the broad keyword Smiley Face T-Shirt, and high on longer keywords like XL Smiley Face T-Shirts, and Red Smiley Face T-shirt. With this approach you would use parameters for items, so you could generate a unique url, but you would canonical it back to the main page.
So the main page would look like this:
url: thetshirtShop.us/mens-tees/smilely-face-tshirt
Title: Smiley Face T-Shirt, Men's Small Through XL
H1: Men's Smiley Face T-Shirt, S,M,L,XL, 2XL, Select A Color & Size
Then for each option you would have your parameters (example red, XL)
url: thetshirtShop.us/mens-tees/smilely-face-tshirt?size=XL&color=red
Canonical: thetshirtShop.us/mens-tees/smiely-face-tshirt
That is the basic structure, but be sure in your content you include the color's and sizes at least twice so when the page is crawled the crawlers see the sizes and colors.
Option 2: which in my opinion is the better of the two would be to create unique content for each item. I know this may seem like it would be difficult or time consuming, and to a degree it is. Again how much different is a red t-shirt from a green one? Aside from the color not much, but there is opportunity here.
Example option 2 color red / xl
url: thetshirtShop.us/mens-tees/smilely-face-tshirt?size=XL&color=red
Title: Red Smiley Face T-Shirt Men's Size XL
H1: Men's Red Smilely Face T-Shirt Size XL
Content Block (Smiley Face T)
By artist Harvest Ross Ball the smiley face t-shirt puts a smile on your chest and a positive attitude by on-lookers. Staying true to the original creator the smiley face tee has been digital optimized and transferred to the tee using our proprietary screen process that insures the image will last. This is one of best selling iconic tees and available in colors, yellow, red, black, white and green. Men's U.S standard sizes, small, medium, large, extra large, and extra extra large.
Content Block (Color-Red):
The T-Shirt Shop using the finest natural dyes in creating our vibrant red color for {Smiley Face T-Shirt}. Using red hibiscus, sumac berries and beets. Our red color is formulated to last for thousands of washes with no fading or wash bleeding. The end result we get true red {Smiley Face T-Shirt} which is quality tested to be HEX #FF0000 RGB 255,0,0 +- within 2 shades.
Content Block (Size XL):
Our Men's Size XL for our {Red} {Smiley Face T-Shirt} follows U.S sizing standards; width 24" (61 cm), length 31.25" (80cm). Sleeve length for the {Red} {Smiley Face T-Shirt} is 9" (22cm), and can accommodate up to a 16" (40cm) bicep comfortably. All our T's are pre-washed and pre-shrunk to ensure our high sizing standards are meet.
Okay, so let me break down what I did here. I created 2 different "specific" code blocks with dynamic elements inside them noted by the { } brackets. Along with the original description block. You can reuse these blocks for other t-shirts, and still achieve a unique page. For example:
Smiley Face T (red/xl) Content Blocks = Smiley Face T, Red, XL
Smiley Face T (green/large) Content Blocks = Smiley Face T, Green, Large
Using these code blocks with dynamic insertion of product details you can achieve a unique page. However you need to get creative in writing these blocks, and do some back end coding or extra work in getting them uploaded to the thousands of different products properly. Just to note I have no knowledge of t-shirts or how best to market them, I just used this for an example to help you identify possible areas of creativity in your product.
In summary I have laid out 2 options for this problem. You can use parameters with canonical and a single page, or get creative and make some unique content pages. Each option will achieve unique urls for your email campaign, and follow SEO best practices.
Hope this helps,
Don