I think this is coming along nicely!  Keep it up.

One possibility I can think (because let's face it - designs in all black are difficult to 'dress up') is having the black envelope be printed with the BC logo - but repeated and glossy.  I'm not sure if I'm describing this correctly, but essentially a relief design (without the costly embossing required).  The black of the envelope is matte due to the paper but the logos are like a glossy clear-coat.

I remember getting a prospectus some years back that was similar and I really though it looked slick.  It gives a contrast due to reflected light but also has a different 'feel' when you touch it.

Still I admit that I have no idea what the cost increase would be in printing them this way... who knows, it might be just as cheap to simply emboss the logo into the envelope paper as this could probably be done at the same time that the 'amount window' was punched.