Are you asking about items you purchase in Frankenmuth or in Ontario? When you enter Canada, the border agents there will want to know what you have with you that could be left in Canada and therefore subject to duty. There are special rules if you're in-transit through Canada. Check the Canada Border Services Agency site at cbsa-asfc.gc.ca/publications/…s2x4.
When you cross back into NY, US Customs and Border Protection will be primarily concerned with what you acquired while in Canada. As long as you have the receipts from items purchased in Michigan, those won't be a problem.
If you're basing your assumption of an $800 exemption on the booklet on the CPB site, please go back and re-read it: cbp.gov/linkhandler/…kbyg_regulations.pdf
You will only be eligible for a $200 exemption as you will not be outside the US for at least 48-hours.