Are your POD locations set up as separate locations? If so, I would reach out to Shopify and ask that your store be moved back to "single origin shipping". When your POD suppliers are set up as different locations, Shopify treats each as its own cart and causes incorrect shipping costs to appear. Since your POD suppliers are the ones who track and manage their own inventory and you have a very simple shipping setup, you only need one origin location and one shipping profile to configure this.
If your support person says they can't move back to a single shipping origin, kindly request to speak to someone else. Reverting back to single origin is still possible, but some Shopify support staff aren't aware it is possible because they're so new to the company.
If you only have one origin location, make sure you only have one shipping profile. Even though your products are coming from different suppliers, your shipping setup is very simple and only needs a single shipping profile.