Is theft/security an issue for you? Mature Christmas trees must be one of the highest-value crops you can steal from a farm, and one of the easiest to fence.
People with 7-8' ceilings cut the top 6' out of a 12' tree bc they didn't like any of the younger trees.
Which doesn't always mean it's the wrong way, from a business perspective. But if you're charging by height, it's leaving money on the table.
If you have the equipment/motivation to steal christmas trees, you probably could be stealing animals too. Those are proabbly worth even more than christmas trees, probably several hundred dollars each.