Fortunately, most people looking at a wine label have no idea whether its a $5, $50, or $500 bottle, so as long as they avoid the mass market supermarket brands like Barefoot, Yellowtail, or Two-Buck Chuck (Charles Shaw), the options in the $10-$15 range are nearly endless. Have them focus more on the type or varietal versus a specific label since they'll want wines that appeal to everyone. If it's only going to be one red and one white, then avoid the really bold styles like Cab. Sauvignon or rich/oaky style Chardonnays, as those can be off putting to some. Having a couple of each, red and white, obviously allows them to cover all the bases more easily. Also, if it's a summertime/warm-weather outdoor wedding, rosé is usually a crowd pleaser. Years from now, no one is going to remember what wine they were served or what they ate at the wedding anyway.