USPS label more expensive via Shopify Shipping than when customer rates at checkout

[LucasSoler]

I've recently noticed that the USPS label is more expensive when I buy it via Shopify Shipping (the Admin) than when a customer pays for the shipping via checkout.  I'm seeing a $2 upcharge per label.

I have the basic plan, so when I (store owner) buys the label, it's using the Shopify account rates, and when my customers buy the label, they're using my company account with USPS.

The thing is, I don't have a negotiated rate with USPS, and I find it hard to believe that my account would get a better rate than Shopify's account with USPS.

This leads me to believe that Shopify is adding a charge on top of the rates they get, when I buy the label from them.  

Anyone else seeing this? I believe my workaround is to either upgrade to Shopify Plus (so that I can use my USPS rates) or use USPS Click N Ship outside the Admin and buy my label there.  Any other options?

[LucasSoler · Accepted solution]

UPDATE:

As Shopify Support just explained, and I verified: the difference in price is due to the "default" box I have selected in the settings, is smaller than the actual box I was using when buying the label via the Admin.  I found a $2 difference.  Thank you Shopify!