This is likely the key to BTC value stability, but at present it's still over the horizon. The number of users continues to increase, the supply of BTC continues to increase. It's still being used as a speculative investment/commodity. Once the number of users becomes more stable (or the growth rate is less jumpy) and the speculation quiets down (to be more like a forex market?), then it'll be feasible for vendors to stick to a price with less risk of getting hosed or hosing their clients.