I am in Canada but lived in the US for a number of years. Had to ship all sorts of stuff back to the family.
If you ship it from USPS (vs UPS or Fedex) its not expensive PROVIDED you fill out this small green form that the post office gives you. So make sure you visit the office vs just putting up the flag for collections. It basically declares that the contents of the box/envelope is not radio active, contains no bio hazzards, no agricultural products, is not a firearm, or simiar nasties. About 4 or 5 little check boxes.
Declare it as a wiring harness and do not put anything on there such as SLOT Machine Part or anything that will attract scrutiny. Canada doesnt have any restrictions on slot machines or parts but as this is not a common shipment you get somebody that says "is this allowed" a meeting is called and 4 civil servents will host a 3 day butt scratching meeting while they make their determination. Ask me what I really think of the Canadian postal service.....

Make sure you put a low-ish value of like $8 on it.
If you go with someone like fedex it changes the 49th parallel into the $49.00 parallel as the item will get held up in customs, you will then get an assigned custom broker which is like a minimum $35 and as much as $65. It will add 15days to the shipping time. Once the value exceeds $50 including the shipping and broker they will then charge inport duties on the item, which is peanuts but it gets the box held at the canadian post office and you have to go and pay the duties there vs ship to your home. You can kind of get around this if you fill out a Proformia invoice (commerical shipping document) - they want the country of orgin (for the harness), serial number (which won't exist), value. Not having a serial number will confuse them. You should then tape 3 copies of this onto the box as well as packing one copy in the box. One agency or another will take this document from the box (thats why you need multiple) and then the next agency won't have it and your back to square one.
I have been through it all..... go with USPS.