how important is reflective film?

I'm trying to set a grow room, only problem shipping reflective film to where i'm at will force me to pay a total of 5 times the original price of the product. Does reflective film to make my grow room walls have a great impact on the growing process or is it something that will give a slight increase in growing?

If they are very important can I use other reflecting materials, mirror walls, silver-colored ducting materials?

Greatly appreciate your advice

Flat white paint is my favorite, put the excess into a good hps, and spare bulb

That's a good idea, it will safe me a shit load of time and money! thank you so much!