Hey Lise.
  Yes, I have asked for lower interest for my visas but was only successful with my Vancity Visa and CIBC Select Visa.  With them, I had to pay a yearly fee (can't remember how much) and was charged a lower rate of interest at around 12% or something.  I had no problems with paying that yearly fee for a lower rate because the fee will pay itself out in a month or two.  It was wayyyy cheaper for me.  But now, I only have one Visa, the Vancity one with the lower rate.
  I tried asking PC Mastercard but they wouldn't budge.  
  What you could do is transfer your balance to PC Mastercard.  They have a very low interest for transfer of balance at around 4% or something.