The issue is that
you are responsible for providing the valuation for the donated item (for smaller items that don't require appraisals), not the organization getting the donation nor the IRS.
This means that while you can list a donated item by weight, you have to be able to justify its value by weight in case of an audit.
Normally, taxpayers value their donated clothing by using valuation guides such as from Goodwill or the Salvation Army (both available online). But these guides require you to itemize the items, at least to the level of "10 mens shirts" or "2 pairs of womens shoes".
Valuation by the pound would make sense only if there was a market to buy such items by the pound (like for rag scrap). Otherwise, you need to come up with a reasonable estimate of what was in that 13 pounds and apply values from the valuation guides to that.
