Here's how flex commissions work: The bank sets a base rate. The dealer can charge you anything up to a maximum rate. The difference is the dealer's commission. The dealer has a financial incentive to charge you as much as possible. You have no idea this is happening.
The best defence: arrange your own finance before walking into the dealership. Get pre-approval from your bank. When the dealer offers finance, compare it to your pre-approval. The car industry is not your friend. It's a business. Treat it like one.