I'm Miles from the Social Care team at Shopify. I'll be happy to help out with your shipping issues. It's important to note that if you're in the United States, then USPS rates are added by default. To ensure you've got the providers you'd like to use, follow the stepsin this guide.
When a customer goes through the checkout, they'll be presented with a list of shipping options to select from. If you've set up USPS, UPS and DHL, they'll see all the options and select one of them. In your case it's likely they've selected USPS as their preferred shipping. If you don't wish to use USPS, my recommendation is to remove it from the shipping options. The guide I linked above should help with that.
When you're ready to ship the order, you might be required toprint the customs formseparately and attach it to the outside of the package.
If the above suggestions don't resolve your issues, we may need to access your account in order to resolve this issue. While we’re not able to provide account-specific support via the Shopify Community at this time, we’d be happy to continue assisting you through live chat, email, or callback. Please visithttps://bit.ly/3cJkx8Vand log in to your account to create a support request.
Miles | Social Care @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog