Fighting a sesatbelt ticket?

i am from manitoba and i just got pulled over a little while ago i had my seatbelt on but i tucked it under my arm. i had just pulled away about 6 car lengths from my parkin and before i got pulled over. i was having arm/ shoulder pain lately and has a profession mage two days ago. when i got pulled over i was asked why i wasnt wearing a seatbelt and why i didnt have a lincence plate on the front of my car i get really nervous when im put on spot and didnt meantion my seatbelt was tucked under my arm i just got a chance to tell him i had my bumper replaced and hadnt had time to put on my front lincence plate he later returned with a 300$ ticket for not wearing a seatbelt. I really dont want to pay this. If i fight this do i have a good chance of winning? what should i do? and are there any other excuses i could say? pleasee help