Here are some reasons that may cause this:

1. Coupons cannot be used on promotional products;
2. The price after coupon is converted based on the original price but not the promotion price when using a coupon.

If the item is on promotion, we kindly suggest you compare the price after using a coupon with the promotion price, and you could click "remove discount" when it is more expensive than the promotion price.
 

hello ， the AI to EOS adapter ring i ssuitable for nikkon AI Mount lens and canon camera body. so you need to confirm your sigma lens specific mount . We care about our customers, any questions, you can ask us directly at customer@kentfaith.com. All emails would be replied in 24 hours, no exception. 

and they look more delicate. 

multi-coated layers,different frame thickness and so on. 

Yes. Camera brand doesn't matter, it's all about the lens diameter.