Memorex DVD+R DL discs are typically RITEKD01, one of the worst DL discs you can find. Many writers have problems writing to these discs and quite a few DVD players have trouble playing them. Ritek cut it's prices on DL media some time ago to gain market share. If they didn't do that, I don't think anybody else would buy those discs.
The people above are being biased - Verbatims are about the only thing you can trust that really works on most writers and DVD players.
Can you shop online? I know there are decent online places that stock Verbatim DVD+R DL discs. I only know of http://www.blankmedia.ca though. If you post in the Blank Media section of this forum, there are people there who can give you more information on where to get those discs in Canada. The people there are also very good at posting information when sales come around, particularly for these very Verbatim discs. Then everyone else proceeds to go clean out the shops with those sales