Get em' to your taxi ASAP!
Here's the deal, the velvet, especially if its still fully intact, has its own blood supply and is like a mini version of a hide over the antlers/bone. Therefore it spoils JUST like a cape/hide. As a taxi, even the process to keep the original velvet on the antlers is a lengthy and detailed chore. If its going to be a day or two before you can get it into the taxi the best way to is keep it cold (do not get wet), and position the horns so they are pointed tines down (hang upside down).....the blood supply of the velvet will run to the tips and keep it from pooling in the bases which really makes the decomp process much quicker. Be very careful with the velvet as well, if it gets rubbed/torn off its extremely hard to repair. Hope this helps and best of luck!!
Joel-BRT