Facts with different unit of measure

I am trying to model a retail sales process in which the grain is a line item in the sales transaction. The items on the transaction could be of different unit of measure like units, Kilos, lbs, boxes...

Should I use one column for quantity in the fact table and have a FK to a UOM dimension with conversions to other units? If I do this though, then this quantity will contain a mix of weights and quantities. Is that a problem?

Or should I create two columns: Quantity and Weight and have 2 FK's to UOM.

Generally speaking in retail, UOM usually comes into play when selling bulk goods (meat, fruit, etc...) otherwise, it is usually just an 'each'. One UOM should be enough, associated with the quantity. If someone buys 2 packages of ground beef, for example, its going to show up as 2 lines in the POS log, each with a different weight. I don't see a need to have both quantity and weight in the fact.

If this is a web retailer, then it would make sense to capture shipping weight as a separate measure.