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A
Nothing — the price is always the same
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B
The package's weight (which you have), dimensions, origin ZIP, destination ZIP, and which service level they need (Priority, Ground Advantage, etc.)
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C
Only the weight
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D
Only the destination
Why this is the answer
Shipping cost is determined by: weight AND dimensions (both affect dimensional weight pricing for larger packages); origin ZIP (determines distance zone); destination ZIP (determines zone); and service level (Priority Mail 1-3 days, Ground Advantage 2-5 days, Priority Mail Express overnight). You already have weight (3 lbs), but you need the other information to provide an accurate quote. Guessing without the full information could mislead the customer.
Source: USPS 477 VEA, Shipping Quote Information