You can order a harness with the breast collar, then but a pair of hames with SHORT TUGS for driving a single animal with a collar. Short tugs are about 12"long from the hames eye, so buckle-in trace is ahead of the harness saddle and not rubbing on shaft. The short tugs have buckles to allow length adjustment on traces or different ended traces for attaching to various style singletrees. Then it is up to you to use the full/neck collar or breast collar with the rest of the harness.
You may want to take the mule to a harness seller, to be sure he is fitted correctly for the full collar. You also may need to get the hames “fitted” to him since most are shaped like this: ( ) insead of a sharper L shape on each side. Your farrier might be helpful there, can heat the hames to bend them in shaping to fit HIS neck. We don’t know any animals that fit the generic shape of new hames. We have modified our hames to fit the horse that wears each set, then marked hames to keep them for that horse.
You might go back in the archives to read about using breast collars and full or neck collars and why folks chose one over the other. Fitting the full collar WELL, and KEEPING it fitting correctly, is difficult on a working animal. He changes shape with muscle use. Full collars are not cheap, you may need a couple different sizes and shapes to keep him comfortable during a season of use.
Full collars do allow pulling a bigger load, but with more skin in contact with the face (inside edge) it is easier to burn his skin when working. You can burn them VERY fast, in less than 15 minutes sometimes! Burn will show with wrinkling skin under the collar, usually shoulders, then peeling within a couple days, often hair loss. Some horses grow hair back white! We don’t use one who has been burned, let skin recover which might be a week or more. Takes a while to get skin under the collar to toughen up, not burn the shoulders. You kind of walk a fine line in getting skin conditioned to work under the full collar, not too much to burn him, not too little or he won’t get toughened.
We have all buckle-in traces on our breast collars. Cheaper to buy traces than a whole new breast collar with sewn-in traces. Same traces fit the short tugs of hames for full collar use.
For a light cart with 2-wheels, a breast collar is fine, and still good for a light 4-wheel vehicle. Full collars are for heavier vehicles, more passengers being pulled. As a beginner driving animal, keeping things light and easy to move for a while, is encouraging to him. He will get more fit, comfortable in his work, so then more weight behind is not “hard work” for him. Often takes a while to get the animal brave with bigger loads, confident in handling it.