You can buy your Citylink 24hr pass up to 3 days AFTER you make the trip. That's better than pre-paying as plans can sometimes change and they will not refund you if you've already paid.

The alternative is to tell your GPS to avoid toll roads. You'll either head down Kings Way, along StKilda Rd and rejoin the freeway at Toorak Rd, or the other way is along the Eastern Freeway and down Springvale Rd.
It's tedious, will take an extra 30-40 minutes (guess!) but will save you $14.25.