I purchased dog food and supplies from Chewy and had my donation sent directly to the pet charity. Which category do I use for this donation?

Since you bought the dog food and supplies for the sole purpose of donating them, simply enter your expenditures as a cash donation, as opposed to contributions of items.

For your own security, just in the event of an audit, ask the charity for documentation that they received the items.  That, in combination with your purchase receipts, should be adequate documentation.