Sarah, you can do it anyway you want. You can wait and when you have $5.00 worth of donation you can send it then, or you can do it per item. It is all up to you. We don't bill you, it is all on the honor system.
If your mom approves, I would highly recommend you setup PayPal, it can really boost your sales, and you get paid immediately, instead of waiting for a check to arrive at your residence, and then giving it at least 10 days to clear. That way your buyers get their merchandise faster, and you get paid without hassles. PayPal is free to setup, and they only take a few pennys from each sale.