You could setup shipping charge rules that would make this charge for you. It would consist of two rules. The first one will be for the first pound at $1.50 and the second rule will be for the additional pounds at $0.50 each.
For the first rule you would give it a name and set the condition to 'subtotal for any column' then select the weight column and specify greater than or =. For the calculation portion of the charge you will specify a flat rate of $1.50.
The second rule will look a lot like the first but it will be triggered on total weight greater than 1lb. For the calculation portion you would choose 'based on multiple of column subtotal', then for the column choose the totalweight then specify times 0.5 minus 1.5. This will add a charge of .50 for each pound minus 1.50 for the first pound.
With both of these rules together you should get the charge you are looking for. The options for basing the charge on the shipping quote value for Canada Post is a mistake, there is no support for Canada Post shipping quotes. I have logged this as a bug so it can be removed to avoid confusion.