Canadian working in the USA (Work Permit)?

I'm a Canadian, working for a Canadian company that has hired a company within the USA to produce a product for us. I'm paid by the Canadian company, and I travel to the United States to go over the functional requirements of our product with the United States company. At the border today, I was questioned about my involvement, and I explained this flow to the border agent, but I'm not sure he fully understood. He asked if I had a letter from my Canadian company, and I handed over the signed letter from our CEO explaining my role in the company as well as the need for me to travel to the United States to oversee work done by our supplier. He granted me the B1 Visa and said the next time I cross the border, I will probably have to provide more documentation or to acquire a work permit otherwise I may be refused entry. I've done some reading on work permits, and it seems to involve being a Canadian getting hired by an American company, which is not the situation I'm in. I'm a Canadian working for a Canadian company that is interfacing with an American company to receive a product by the American company. Based on the flow that I've explained, could someone please provide a source that goes over all the different types of work permits/Visas and what type of documentation I should always have with me at the border? (Such as Bank statements, recurring bills, Canadian Rental/Lease/Mortgage agreements, other?)