It depends on the state. In most states you have to purchase Comprehensive coverage that has Full Windshild or Full Glass Coverage in the deductible section. Some states don't offer this at all, some states (Florida for example) include windshild coverage when Comprehensive coverage is purchased (regardless of the deductible option selected). Under most circumstances your front windshield is covered if you have full coverage.
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Typicall, the term full coverage includes comprehensive and collision.
With full coverage, in some states you do not have a deductible for the front windshield, but all other windows are subject to the deductible.
If you only have liability insurance on your vehicle you will have to pay out of pocket for the replacement of your windshield, since it is not covered by your policy.
Comment Update: Your math of $250 + $350 = $600. The easiest way to understand the deductible it this, in the event of a loss you are responsible for the FIRST $250 (the deductible) and the policy will pay the rest. [Let CarInsurance.com help you find affordable auto insurance now.] | 
