Woocommerce, Volume discount by eg six

Hi all,

I am currently helping a friend setting up a small online wine shop. I have only done one WP site before and not with Woocommerce.

Wines on the site are sold piece-wise or in boxes of eg six. If the customer buys a box of six there is a discount of eg 20%. If a bottle has a price of 50 and a customer buys 7 the total price is then 50+6*50*0.8 = 290.

Now, this can be done using variable product with variant A to 50 and variant B to 240. The problem is then to the manage stock. If we have 600 bottles they can both be sold as variant A (600 products) and B (100 product) but B should subtract the stock amount by 6 and not by one. Is it possible to link the two variants to the same stock amount?

I could also use a discount plugin and create one simple product but I have not found a suitable one. The all support volume discount like if >= 6 then discount in steps of 1 but I need steps of eg 6.

Maybe some of you have struggled with this issue? or know of a good plugin I can checkout.

https://wordpress.org/plugins/woocommerce/