Why did my order ship through a different carrier/method than I selected?

Typically, this occurs with USPS Priority when a large order is placed and shipping is undercharged due to the order hitting a dimensional weight threshold that Shopify cannot calculate. After box dimensions are input during order processing, the rate increases due to the size of the box. Usually, these orders can be swapped to UPS Ground or Fedex Ground for a better rate than USPS to avoid additional charges.