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.
 

The cover is for 62 mm optics. Just measure what the Lumix has. 

That depends on the lens diameter. This is usually indicated on the lens. The filter holder is then attached to the lens with the appropriate adapter ring 

and they look more delicate. 

This will work on any 82mm lens.