We would advise getting your visa in advance. When we took the UA trip in 2009, the line for getting visas at Victoria Falls was so long we'd still be waiting on it. Fortunately everyone in our group had their visa in advance so we were long gone before the first person on line for those with no visas was processed.
If you're going in a group, which you likely will be if this is an extension, you will not endear yourself to your fellow travelers if you hold them up because you didn't get your visa in advance. Our advice: Be a good citizen and get your visa in advance and don't take a chance that you'll be the cause of making your group wait for you. If you don't want to pay the fee for PVS, which nanaandpapa have indicated, you can go directly to the consulate yourself, either by mail or in person.