On fixed priced GC? Tell them they are full of it and ask to see the manager. I have heard that about many a card there, like ReloadIt and the Univision and free cards, but the fixed priced 100/200 VGC HAVE ALWAYS been allowed. Ask again - it should be fine.
The main con to this idea is the low denomination (IE a pain to unload, unless u can use evolve) and the fact that safeway sells really expensive priced food, so the 4 bux you save does not go a long way - barely break even, but it works.