A free extension by Iain at Polevaultweb which allows you to set a simple initial shipping price for every product. The price can be reduced for multiple items up to a maximum number of products.
So if you’re selling a product you can set your standard shipping price at £2, but have it drop to £1 for any additional product ordered up to a maximum monetary value. For instance, if you set the value at £5, the product cap will kick in at five products (£2+£1+£1+£1).
The download is offered via the WordPress repository, with support provided by Polevaultweb