Subproducts, attributes and variations - Questions and Requests

I've installed the ecommerce module and wanted to address a few apparent limitations...

Variation Surcharge by Percent:
I have the need to calculate a surcharge by percent. We sell products of varying prices that are available in a wide array of colors, including some films (non-solid colors) that are an additional 15%, (a $30 product with this special film would be $34.50 and a $300 product would be $345). It appears the only option available is to apply a set dollar amount. How could I add a surcharge by percent?

A quick question:
Is the only way to have subproducts to use Apparel? My products are not apparel, but that is the only way I've been able to add subproducts. Is there a way of adding a custom product module, basically a clone of apparel? Or should I just use apparel for motorcycle parts? :)

Two "Add to Cart" buttons?
When I add variations, such as colors, I get two "Add to Cart" buttons, one in the variation box and one under the quantity amount. The one in the variation box functions correctly, but the one under the quantity gives me the following error: "Unable to find the correct product based upon the variation combination." I selected the "hidden" button under "Add to cart link", but it still shows up. Any ideas? I suppose this is a bug?

Also, when generating subproducts, I receive the following error for each variation:
warning: array_filter() [function.array-filter]: The first argument should be an array in /home/content/m/a/i/maierwerx/html/jessemaier/x/sites/all/modules/ecommerce/contrib/ec_roles/ec_roles.module on line 126.
warning: array_filter() [function.array-filter]: The first argument should be an array in /home/content/m/a/i/maierwerx/html/jessemaier/x/sites/all/modules/ecommerce/contrib/ec_roles/ec_roles.module on line 130.
warning: array_intersect_assoc() [function.array-intersect-assoc]: Argument #2 is not an array in /home/content/m/a/i/maierwerx/html/jessemaier/x/sites/all/modules/ecommerce/contrib/ec_roles/ec_roles.module on line 137.
warning: array_intersect_assoc() [function.array-intersect-assoc]: Argument #2 is not an array in /home/content/m/a/i/maierwerx/html/jessemaier/x/sites/all/modules/ecommerce/contrib/ec_roles/ec_roles.module on line 138.
warning: array_intersect_assoc() [function.array-intersect-assoc]: Argument #2 is not an array in /home/content/m/a/i/maierwerx/html/jessemaier/x/sites/all/modules/ecommerce/contrib/ec_roles/ec_roles.module on line 139.
warning: array_intersect_assoc() [function.array-intersect-assoc]: Argument #2 is not an array in /home/content/m/a/i/maierwerx/html/jessemaier/x/sites/all/modules/ecommerce/contrib/ec_roles/ec_roles.module on line 140.
It seems to work fine, but I am unsure what the error message means. Any insight?

Finally, this may not be an ecommerce question, but it goes along with it so I'll ask it here:
I am having trouble figuring out how I would correctly categorize my products with Drupal. I sell products that fit a particular make/model/year. I can use taxonomy and views to categorize my make/models, but then I'd like the user to be able to select a year. The problem is, some models might be 1990 - 1998, while other models are 2004 - 2008. If I use an exposed filter for years 1970 - 2008 it gives them a bunch of years that don't apply to that model. How could I allow the user to select a year for a particular model from a list of years that only applies to that model? Does that make sense? For instance... ATV=>Honda=>TRX450R =>2008 (Years available: 2004-2008) or Street=>Suzuki=>GSXR750=>2005 (Years available: 1995-2008)

I appreciate any guidance!