In THEORY there is no problem to cross the border between Poland and Ukraine on the road with own vehicle.
On praxis it can be extremely time-consuming and as I noticed, tourists from "Western" Europe have little ideas how to overcome the red tape. Tourists with USA or Canadian-registered vehicles are even bigger rarity and neither Polish nor Ukrainian authorities have sufficient experience, how to clear cars with American plates.
I did it successfully - in the fact, my US-registered car was whizzing past other, less lucky locals(!!!) and even less lucky tourists from Netherlands, but I had some luck, too.
Pay attention to:
- crossing the border is best on weekends
- ask local police in Poland or Ukraine, what is the situation on the border. Sometimes queues can be 13 hour long! This can be exhausting, in particular if with children.
- have valid "green card" for Ukraina. Without it you will have to buy the obligatory third-party liability insurance at the border. It is not expensive, but the process adds maybe an other hour.
- I met some customs officers on Ukrainian side who lived in the States earlier. To expats: make sure all the usual State stickers are in place, expect the Polish or Ukrainian control knowledgeable enough - appearing with expired State registration stickers and/or expired registration paperwork is a very bad idea. This is generally true all over Europe: once I met a Croatian border guard whose previous job was at CHP (California Highway Patrol).
- With some exceptions (see above) Ukrainians are used to European registration paperwork only. They know, however how an American title looks like and having both original title and Bill of Sale will expedite the entire border crossing procedure enormously. As a tourist you do NOT need to pay any taxes, "advance fees" or such and I did not experience, Ukrainians or Poles would demand them either.
- Without "Green Card" valid for Ukraina or a special Ukrainian-Belarus-Russian "lesser Green Card" your car can't leave the fenced border zone, no exceptions. The procedure is to park the car somewhat outside of the immediate border control area, take the white control paper and car's paperwork with you and walk out of the border zone. On some border crossings you may walk as far as a mile till you see the booths selling insurance. It's not a very pleasant walk because of the trucks entering-leaving the area. The Ukrainian insurance is very reasonably priced. I have "green card", so I did not need to buy it. I still asked and I was quoted less, than $40 / year. Without either "G/C" or that insurance it's illegal to drive ANY car in Ukraina. With insurance paperwork walk back to the car, show the paperwork to the border guards, they will stamp the "control coupon" and you can drive out of the border zone.