I usually do it per head on everything, or if someone tells me what they need I just give a price to get them what they want, of course I'm still figuring it by volume. I'm also not that big and the per head deal just keeps it simple.
The order buying portion of our business charges $1/cwt. We only deal in semi loads. Frankly, we only maintain a handful of large customers in addition to the buying needs of our own family's ranches and cattle feeding.