The add-on rewards allow you to customise your reward to receive any combination that you want. Once you have decided what garments you would like you can select the reward that works best. Here are a few examples of how this could work:
If you want 1x Nomad Crew and 2x Kailash ¼ Zip then you would select the Nomad Crew reward NZ$135, and then add on NZ$250 (NZ$125 each) for the two extra Kailash ¼ Zip garments. This would bring your total to NZ$385, which you will need to manually adjust.
If you want 2x Namtso Tee, 1x Nomad Crew, and 1x Kailash ¼ Zip, then the cheapest option is the Complete Kit Bundle + an add-on Namtso Tee. This means you would select The Complete Kit option for NZ$310, and then add-on NZ$80 for another Namtso Tee. You would need to manually adjust the total to be NZ$390.