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.
 

Could you please update us your full Minolta lens model ?as Minolta has MD MC and AF mount, K&F Concept MD-M4/3 is suitable for: Lens: Minolta MD MC Mount lens(not for Minolta AF mount: KF06.157) Camera: Micro 4/3 (MFT) mount mirrorless camera like Olympus PEN, OM-D series, Panasonic Lumix DMC-G1, G2, G3, G10, GX1, GH1, GH2, GF1, GF2, GF3, GF5, GH4, Panasonic AG-AF100 Mirrorless Digital Cameras, Black Magic Pocket Cinema Camera (BMPCC) 

and they look more delicate. 

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

Please note: our M42 adapter ring could not be suitable for Pentax Takumar Super Takumar Carl Zeiss M42 series（aperture pin does not fully depress, small tabs sticking out), as ours depresses the aperture pin all the way down