Ultimately its up to you and your exact requirements. But here are following suggestions:

1. SKUs does not exists if Products does not exist. In other words, you cannot make a SKU as a standalone entity. The basic entity therefore is Products. And as a Product can have multiple SKU, make this relationship many to many.
2. So if you follow point 1, You can safely remove shop_skus relationship because now, a shop can select products via shop_products and as products are in many-many relationship with SKU you can enable shop to pick suitable SKU directly using suitable query.

Edit: I didn't notice that this has been suggested already in comments by ypercube. So crediting him here.

Ultimately its up to you and your exact requirements. But here are following suggestions:

1. SKUs does not exists if Products does not exist. In other words, you cannot make a SKU as a standalone entity. The basic entity therefore is Products. And as a Product can have multiple SKU, make this relationship many to many.
2. So if you follow point 1, You can safely remove shop_skus relationship because now, a shop can select products via shop_products and as products are in many-many relationship with SKU you can enable shop to pick suitable SKU directly using suitable query.