Hi,
The "price per product" have two kind of options in the product page : The price and the fee.
The "price" is the product for each product and the fee is a global price of all.
Using "fee" and quantity, you can set "qty 1 / fee $3" and "qty 3 / fee $7".
But it only works per product, not for different products in the same category.
So, if you just set values in "fee", the price will be dependent of the quantity for the product.
I think you need a global quantity price in the same category of product and at this moment, the new shipping system that is currently in development will help you because it will allow you create shipping groups and have different shipping configuration for these groups.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.