i have heard that Kenmore has weekly last-minute specials but have never found them online.
Kenmore/Clipper now offer a package at kenmoreair.com/Sea-Sky-Seattle-Victoria-Kenm… that flies you one way and catamarans you the other way. Can't tell if this would actually save money vs separate bookings, so check it out.
You should have no weather problems at the end of August ... unless a seaplane terminal gets fogged in, which is a pain if it happens, as who knows when the fog will lift. It might be a good idea to fly the first leg. Then if it is fogged in, you can switch to the Clipper, and hopefully fly back without problems. My last Kenmore trip was fogged in, and after several subsequent flights were cancelled they just zipped us down to the Clipper terminal, and credited us generously for the fare difference :-)