setting an item up for assembly

Hi,

This is causing me so much trouble. Please help.

I buy larger bags in 15kg or 18kg sizes and break them down to as small as 100g units to sell as part of other items.

I already figured out that my big bag had to be set up as a different sales unit than purchase or stock unit and when I build my smaller items it worked beautifully but then I tried to create an invoice where a full bag is sold and the system went crazy

My set up was like this

Stocking unit 15kg

Selling unit

unit of measure 100g

Relationship 150   100 per 15kg

Buying unit is same as stocking unit.

I can build 250g, 2kg and 4kg new units no problem. Unit price calculates properly, inventory updates properly

but when I enter to the invoice I have a choice to choose 100g units or 15kg unit

I choose 15kg (I want that to show on the invoice) and enter quantity 1. The system than calculates the prices as 150 times the full bag price. 

what am I doing wrong? I have been at it for over a week.

Thank you