How do I adjust orders for items sold by weight?

The by far easiest solution is to have the grower sell at a flat price. Pick an average cost, and charge everybody the same, even if the weight fluctuates a little.

If the grower has a large range of sizes, then he can separately list small, medium, and large roasts (for example), each with their own set price. Just be sure the description has the weight range explicitly specified, so the customer knows what they're getting.

Take a look at my market (http://athens.locallygrown.net/market) for examples. We move thousands of dollars of meat a week, including beef, pork, goat, chicken, and lamb, all at fixed prices. The customers prefer it this way, the growers make the same money in the end with less work, and the managers & volunteers are saved a ton of work.

I have never gotten a single complaint from my customers, in nine years of doing this, that the price per pound is slightly higher or lower than another item, so long as the items were actually in the stated weight range and the range itself wasn't too broad.

Selling 3lb and 5lb chickens for the same flat rate would not be good, but having a 4 to 5lb range is perfectly fine. Calculate the price for a 4.5lb bird, and use that price. Some people will get a bird that might way up to 8oz less, and others up to 8oz more, but it all averages out over time.

Just be sure you actually have products in the range specified. I could see an unscrupulous vendor setting a 4 to 5lb range, setting the price at the 4.5lb weight, and then bringing in nothing but 4lb birds. I've never experienced that, but it's something to keep in mind. Just like bags of lettuce that are skimpy, or any other item really.

If you can't sell it this way, then be prepared to have to go back into each order and change each price to reflect the total cost. If you can do this before printing out all of the invoices, that's great, but you'd probably not know yourself until the grower brings you the items. Also be prepared for a sizable number of customers to get "sticker shock" and refuse to purchase items they had ordered. Both of those problems go away when you sell at a flat rate.