Shipping Label Prices Fluctuate

[war1867]

I've sold dozens of items using the same packaging each time, and so the packaging has consistently been the same dimensions as well as weight. However, when printing out a shipping label, the prices of this fluctuate constantly. Sometimes it will cost $2.76 to send an item out, and other times it will cost $3.18, for an item that was the exact same weight and dimensions as my other items that have cost less to ship. Why does this fluctuate? Shouldn't it always be the same price if my package weight and dimensions haven't changed? I'm very confused, thank you for any insight. 

[nobody*s_perfect]

USPS postage prices have 9 Zones.  Here are the current rates for First Class packages:

[soh.maryl]

Besides weight and shipping class, the fee for shipping takes into consideration the distance.

Are you always shipping items to the same area? 

[war1867]

Thank you! This clears it up much better!