In HV, the company Mattamy uses was Mirror Interiors (
www.mirrorinteriors.com ). They have a show room close to Winston Churchill & Dundas. What we did is after-closing we visited them, and negotiated prices for them to come and do our shower door, and 3 mirrored-sliding closet doors for us. We saved a modest amount by doing it directly through them versus through Mattamy. Although we had to negotiate with them on the pricing a bit.
See if you can find out what vendor mattamy uses in Summerside for their glass shower doors. Then check out what their retail prices are like.
Sometimes you have to be careful because if you say you're buying a mattamy home some of Mattamy's suppliers won't talk to you until after closing about pricing.
After you work out taxes, installation costs, etc, the mattamy price on the shower door wasn't that much different (just slightly more) than us shopping around ourselves.
We didn't go with the frameless door though, so ours wasn't as expensive.