No, you weren't clear about your nationality, and the specifics of this little cross border shopping adventure. Are you in Ottawa? Name now sounds familiar.
I would expect you'll have to pay GST at CBSA, and HST possibly at Service Ontario/MTO, just like any other item, in this case low speed vehicle, you purchase outside Canada, and want to bring home.
I would start with Transport Canada's website to determine whether or not you can bring the golf cart in legally, and under what restrictions. Just like any other importation of a vehicle from outside Canada. btw, travel trailers (and probably 5th wheels) must also be cleared as "importable" by TC. I brought one in from Michigan back in 2013, and had to get it cleared for import, and then inspected here, and approved, before it could be licensed in Ontario/Canada.
IIRC, the US side of things was handled by the RV dealership. If there were export controls forms required, they took care of them. Can you ask the golf cart seller about it? I don't recall stopping on the US side when we brought it through on the Bluewater Bridge. We did have to stop at CBSA secondary with some paperwork. Then home. Then Canadian Tire to get the inspection done, then MTO for registration.
As for golf carts, I'm less certain, and they may be considered a "low speed vehicle" under the Ontario pilot program guidelines, and will be regulated at the municipal level for legal usage.
https://www.ontario.ca/page/low-speed-vehicle-pilot-programIn hindsight, unless there's a pressing need to bring it home, can you leave it at your campground or with someone else down there? Might be the easier route, at day's end.
Mike. Comments are anecdotal or personal opinions, and worth what you paid for them.
2018 (2017 Sprinter Cab Chassis) Navion24V + 2016 Wrangler JKU (sold @ ????)
2016 Sunstar 26HE, V10, 3V, 6 Speed (sold @ 4600 miles)
2002 Roadtrek C190P (sold @ 315,000kms)