The reason they need the minimum is they have upfront costs with the tank. They are not cheap so if you don't buy a minimum amount of fuel they have huge carrying costs and it would take them many, many years to recoup the cost. The tanks aren't free.
My builder got me my tank for my gas F/P at cost (120 gallon) and it was around $400 bucks for the tank and regulator (somewhere close to that number I don't recall exact price).
In order to be able to shop for gas prices your only option is to buy your own tank and cough up the money upfront as was mentioned above.
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