Where a product fee applies, it will be added to your new mortgage. You can then pay the fee off if you want to, or leave it on your mortgage to spread the cost. If you pay it off within 30 days of the start of your mortgage, no interest will be charged on it. If you leave it on your mortgage, interest will be charged on it as part of your main mortgage. As this will increase your loan amount, it will also affect the monthly payment on the main part of your mortgage.