You can define Units of Measure of a product in your Inventory catalog. 

Under Management, select Inventory and then Catalog from the drop down.

Select the blue plus sign and then click New Product 

OR Click on an already existing item.

You will see a Units of Measure field (UOM).

Under value you can indicate the applicable multipliers for this product.

The UOM defined here will be used to calculate the CHARGE amount and the QUANTITY when creating an invoice. 

Charge = (Item delivered * UOM) / Billable Units * Price Option

Quantity = (item delivered * UOM) / Billable Units 

Billable Units are defined on the price option level

Example: A single product is purchased that has a quantity of 2 in the package.You sell this product and need the invoice to display your charge multiplied by 2 and your quantity multiplied by 2. 

In the UOM field you would put a value of 2

In the billable units under the price option you would put a value of 1