Oh, that's easy. The BoC overnight rate is only one of the factors that go into the Prime Rate, which is determined by the banks themselves. The Prime Rate is also down by about half a percent.
Credit card rates on the other hand, are set by the banks based on how much they want to rip you off. The only government involvement there is that the card has to stay under the criminal interest rate, or 48% APR.
The current Government has proposed to reduce that rate to 35% APR, but we'll see.
In short, your MP won't be able to help with your credit card, because cards are issued by the banks, not the Government. Personally, I'd love to see Canada Post get into personal banking, but it's a bit of a pipe dream.
Genuine curiosity, why would Canada Post get into banking? Do they have some administrative advantages that would help, or is there no better department?
Postal banking is a thing in many countries. Canada Post did banking from its inception until 1968. The major benefits are that there is a post office in every community, even really tiny ones, and that a Canada Post bank system can offer basic banking services to people who otherwise wouldn't be able to.