Sunflowers: When to harvest & post harvest practices to extend blooms 1-1.5 weeks out

My main venue for selling is through a farmer's market that occurs every other Sunday. It can be tricky to time blooms every other week and since sunflowers are the "backbone" of my market, I was highly motivated to figure out: 1) When to harvest and at what bud stage to buy myself the most amount of time for a market that is over 1.5 weeks out 2) What temperature to set my fridge at to prevent "fridge burn" but also slow down opening 3) What stage should I expect my sunflowers to be at during market day based on when I harvest/stage?