Why does the cost of USPS Priority Mail shipping labels vary using Shopify Shipping?

[ASeller]

The cost of Shopify Shipping Priority Mail Padded envelope labels varies, and we want to know why.  We purchased two Priority Padded shipping labels within two minutes of each other, and one cost $9.30 and the other cost $8.95.  Both labels were to mainland US addresses, and were NOT to Hawaii or Alaska, and weight is not a factor in Priority mail under 70 pounds.

Why is the discount given by Shopify Shipping different on some labels?  

We need consistency in label cost so we charge for shipping correctly in our shop.