Others who know more about it will be along soon, I imagine, but from my small foray into this a few years ago..
There are regs about temperatures and so on, and freezing before selling is problematic because the rate of freezing in domestic freezers means that most of us would struggle to freeze several carcases down quickly enough, not to mention that if the packs come home before going out then you may have to demonstrate that you comply with food handling regs, max temperatures, etc.
So the option I went for, which worked well for me, was to get the boxes, labels, cool packaging and so on to the butcher, the butcher packed up, labelled and sealed the boxes, then the courier picked them up direct from there. Overnight carriage with next morning delivery keeps the meat cool enough (although I discovered that paying extra for "by 10 o'clock" did not achieve "by 10 o'clock" in all cases, due to the distances involved, so would go for "by noon" in future), then the customers can freeze things themselves.