woocommerce-onlineShop: first 50 Items for free

Hi!
We′re offering brochures and our customer can order up to for example 150 items/product.

We offer the first 50 items of each product for free, so if a customer orders a quantity of 60 from a single product, we would like to calculate just 10 of them, while the costs of delivery still will be calculated with the weight of the 60 (using tableRatePlus and it works).

Any ideas?

Thanks and Regards

https://www.ads-software.com/plugins/woocommerce/