Ok - we are wanting to switch from flat rate shipping to letting Shopify calculate the actually shipping charges on the fly for each order.
When I go into setting and add "Calculated Rates" and then choose DHL, USPS and UPS (and our UPS account is setup and linked) the only one it is showing to the customer when they go to checkout is USPS - why aren't DHL or UPS showing up?
Can you tell me which Shopify subscription plan you are currently using?
You need to have the carrier-calculated shipping feature on your store's Shopify subscription plan to connect your own shipping carrier accounts to Shopify. This feature is included in the Advanced Shopify and Shopify Plus plans and can be added to any plan for a monthly fee.
Few shipping apps which are integrated with multiple carriers like the multiple carrier shipping label app will automate the display of carrier calculated shipping rates from all the available carriers like USPS, UPS, DHL, etc, at the checkout, and also helps to enable the carrier calculated shipping feature in a simple way.